Upgrading a classic Small Block Mopar (SBM)—specifically the 318, 340, or 360 LA series engines—to a modern Tremec T56 Magnum six-speed manual transmission is one of the most transformative modifications an enthusiast can perform. This conversion replaces the archaic A833 four-speed or the lackluster 904/727 automatics with a gearbox capable of handling 700 lb-ft of torque while providing two overdrive gears for modern highway cruising.

However, the T56 Magnum is not a "bolt-in" replacement in the traditional sense. Because of its massive casing and modern internal design, fitting it into an A-body Dart, a B-body Charger, or an E-body Challenger requires a specialized conversion kit and significant structural awareness. This article analyzes the essential components of these kits, the technical hurdles involved, and how to select the right configuration for your driving style.

The Engineering Advantage of the T56 Magnum Platform

Before diving into the kit components, it is crucial to understand why the T56 Magnum is the preferred choice over the standard T56 (from a Viper or LS1 Camaro) or the smaller TKX five-speed. The Magnum is essentially the aftermarket version of the TR-6060 found in the Hellcat and GT500.

  1. Torque Capacity: Rated at 700 lb-ft, it comfortably handles stroked 408ci or 410ci small blocks that produce high-RPM power.
  2. Shift Quality: Featuring triple-cone synchronizers and a short-throw internal rail shifter, it eliminates the "clunky" feel associated with older Toploaders or Muncie transmissions.
  3. Versatility: It offers multiple shifter positions, which is critical when trying to line up with a Mopar factory console.
  4. Double Overdrive: With a 0.63 or 0.50 final drive ratio, you can run aggressive 3.91 or 4.10 rear gears for stoplight acceleration while still turning sub-2000 RPM at 75 mph.

Core Components of a Small Block Mopar Conversion Kit

A comprehensive conversion kit from specialized vendors like Silver Sport Transmissions (SST) or Modern Driveline (MDL) typically includes more than a dozen critical parts. Relying on a "piecemeal" approach often leads to compatibility issues, particularly with pilot bearings and clutch stack heights.

The Specialized Bellhousing

The LA small block (and the later Magnum 5.2/5.9 engines) features a specific bolt pattern that differs from the Big Block Chrysler or the Gen 3 Hemi (though some modern bellhousings are dual-pattern).

For a T56 swap, you have two primary choices:

  • Cast Aluminum Bellhousings: Brands like American Powertrain offer high-quality aluminum castings. These are lighter (approx. 33 lbs) and more budget-friendly. They are ideal for street-driven cars where weight savings are prioritized over racing certifications.
  • Steel SFI-Rated Scattershields: QuickTime (Holley) produces steel bellhousings that are mandatory for many drag racing classes. They provide a safety barrier in the event of a catastrophic clutch failure. In our technical assessment, steel bellhousings are slightly more difficult to align (dial-in) but offer peace of mind for high-horsepower builds.

Flywheel and Clutch Synergy

The Mopar small block uses a 130-tooth flywheel. When ordering a kit, you must specify if your engine is internally or externally balanced.

  • 318 and 340 (Pre-1973): Usually internally balanced, requiring a "neutral" flywheel.
  • 360 and 340 (Cast Crank): Externally balanced, requiring specific weights on the flywheel or a balanced pressure plate assembly.

The clutch disc must be a 26-spline unit to match the T56 Magnum input shaft, which is a departure from the 10-spline or 23-spline shafts found on vintage A833 transmissions. A 10.5-inch clutch is the standard for these conversions, providing a balance between holding power and pedal effort.

Hydraulic Release Systems

Vintage Mopars utilized a mechanical "Z-bar" linkage. The T56 Magnum is designed for hydraulic actuation. Attempting to adapt a mechanical linkage to a T56 is an exercise in frustration. A quality kit will provide:

  1. Hydraulic Master Cylinder: Often with a custom mounting bracket that uses the factory firewall holes.
  2. Concentric Slave Bearing (CSC): This slides over the input shaft and replaces the traditional throwout bearing and fork.
  3. Steel Braided Lines: To handle the high pressures required for a crisp clutch release.

In our field testing, the most critical step here is the "shim measurement." You must measure the distance from the fingers of the pressure plate to the face of the transmission to ensure the CSC has the correct air gap (usually 0.125" to 0.175"). Incorrect shimming will result in a clutch that won't disengage or a bearing that burns out prematurely.

The Reality of Tunnel Modifications

If there is one aspect of the Small Block Mopar T56 conversion that intimidates owners, it is the sheet metal work. The T56 Magnum is significantly taller and wider at the rear than the factory transmissions.

Why You Must Cut the Floor

In an A-body (Dart/Duster/Valiant), the factory tunnel is notoriously tight. To achieve the correct driveline angle (usually 3 degrees downward), the T56 must sit high enough in the chassis that the top of the case will interfere with the original floor pan.

Standard kits include a "Tunnel Patch" or a pre-formed sheet metal hump. The process involves:

  1. Marking and Cutting: Removing the center section of the floor from the firewall back to the torsion bar crossmember.
  2. Bracing: Some kits require a modified transmission crossmember that provides structural rigidity after the floor has been cut.
  3. Welding or Bonding: The new, larger tunnel is welded into place, providing the necessary clearance for the 6-speed's "shoulders."

For E-body (Challenger/Cuda) and B-body cars, the clearance is slightly better, but "tunnel massaging" with a mallet or minor cutting is still frequently required to ensure the transmission doesn't vibrate against the floor under load.

Gear Ratio Selection for the Small Block

The T56 Magnum comes in two primary gear sets. Choosing the wrong one can make your Small Block feel sluggish or excessively "geared out."

The 2.66 First Gear (Wide Ratio)

  • Best for: Cars with 3.73, 3.91, or 4.10 rear gears.
  • Characteristics: Provides a longer first gear, which is useful if your SBM makes a lot of top-end power. The 0.63 sixth gear is perfect for high-speed highway travel.

The 2.97 First Gear (Close Ratio)

  • Best for: Cars with 3.23 or 3.55 rear gears.
  • Characteristics: The steeper 2.97 first gear helps the car get off the line more easily, compensating for taller rear-end gearing. This is often the preferred choice for street cruisers and heavy B-body cars.

Body-Specific Conversion Nuances

A-Body (1960–1976)

The A-body presents the most challenges due to the narrow frame rails and small tunnel. When installing a T56 kit here, you must be extremely precise with your engine mounts. Even a 1/4-inch offset can cause the transmission to hit the steering column or the floor. Most A-body kits require a custom-designed crossmember that maintains the factory torsion bar suspension.

B-Body and E-Body (1966–1974)

These cars have more lateral room, but the shifter location is the primary concern. To keep your factory console, you often need a "mid-shift" conversion or a shifted offset handle provided in the kit. If you have a bench seat, you will need a forward-position shifter to avoid the seat cushion when shifting into 2nd, 4th, and 6th gear.

Driveline and Speedometer Integration

Once the transmission is physically in the car, two final hurdles remain: the driveshaft and the speedometer.

  1. Driveshaft: The T56 Magnum uses a 31-spline output shaft (similar to a Ford Toploader or C6 automatic). You cannot reuse your Mopar 727 or A833 slip yoke. Most kits include a new 3.0-inch or 3.5-inch aluminum driveshaft balanced specifically for the T56's output.
  2. Speedometer: The Magnum provides both a mechanical cable drive and an electronic VSS (Vehicle Speed Sensor) signal. This is a huge advantage. If you have vintage gauges, you can simply plug in a standard mechanical cable. If you have upgraded to a digital dash, the electronic signal is ready to wire in.

Installation Tips from the Shop Floor

  • Dial Indicator is Mandatory: Do not assume the bellhousing is centered. Use a dial indicator to check the "run-out" of the bellhousing relative to the crankshaft. If it is off by more than 0.005", you need offset dowel pins. Failure to do this will cause the T56 input shaft to bind, leading to catastrophic bearing failure within 500 miles.
  • Pilot Bushing vs. Bearing: Many Mopar crankshafts are not drilled deep enough for the long input shaft of a T56. You must measure the crank bore depth. In some cases, you may need a "shorty" pilot bearing or you may need to trim 1/4-inch off the tip of the transmission input shaft.
  • The Reverse Lockout: The T56 Magnum features an electronic reverse lockout solenoid. This prevents you from accidentally shifting into reverse when looking for 5th gear. A proper conversion kit will include a wiring pigtail or a "reverse lockout module" that allows the solenoid to open when the car is below 5 mph.

What is included in a Small Block Mopar T56 Magnum kit?

A high-quality kit should include the T56 Magnum transmission, a specialized bellhousing (aluminum or steel), a 130-tooth flywheel, a 26-spline clutch and pressure plate, a hydraulic master and slave cylinder system, a custom transmission crossmember, a 31-spline slip yoke or full driveshaft, and all necessary Grade 8 hardware. Many kits also include a tunnel patch for floor modifications.

Do I have to cut my floor to fit a T56 Magnum in a Mopar?

Yes. In almost all classic Mopar applications (A, B, and E bodies), the T56 Magnum is too large for the factory transmission tunnel. You will need to cut the center section of the floor and install a higher-profile sheet metal tunnel patch to allow the transmission to sit at the correct angle without interfering with the chassis.

Can I use my original clutch linkage with a T56 swap?

It is highly discouraged. The T56 Magnum is designed for a hydraulic concentric slave cylinder. While some custom adapters exist to use a mechanical fork, they are prone to geometry issues and high pedal effort. Modern hydraulic kits provide a much smoother, lighter pedal feel that is more compatible with the T56's internal design.

Which gear ratio is better for a 360 Mopar?

If you are running a 3.55 rear gear, the 2.97 first gear (TUET11012) is generally better for street drivability. If you have 3.91 or shorter gears and a high-revving 340 or 360, the 2.66 first gear (TUET11009) will allow you to stay in gear longer and make better use of the engine's power band.

Summary

Converting a Small Block Mopar to a Tremec T56 Magnum is a significant undertaking that requires a blend of mechanical assembly and metal fabrication. By utilizing a comprehensive conversion kit, you eliminate the guesswork regarding clutch spacing, bellhousing alignment, and driveline geometry. While the requirement to cut the floor remains a hurdle for some, the reward—a bulletproof 6-speed that offers both blistering performance and effortless highway cruising—makes it one of the most valuable upgrades available for the Mopar platform. Whether you are building a Pro-Touring Cuda or a high-performance Dart, the T56 Magnum is the definitive solution for those who refuse to compromise on strength or shift quality.