There are several considerations when swapping a late trans into an early car including, but may not be limited to the following, depending on exact car-engine-trans combination:
Some of the pre-55 car/engine assembly may have rear engine mounts attached to the bellhousing. The new trans mount will be about a foot farther to the rear so a new crossmember will be required. New engine mounts should be fabricated so as to attach to the two bolts that secure the plate to the block on each side of the engine, and an additional transmission mount must be installed.
The late starter requires 12 volts. Please refer to the ‘Electrical’ section for additional information. You will no longer have an parking/emergency brake. The common fix is to replace the rear axle. Suitable replacement units can be found by comparing the width of the wheel mounting surfaces. At this point you also have the option of using either drum or disc brakes, depending on whether or not you install disc brakes on the front.
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Spring pads are readily available for welding to the new axle housing.
A new cable will be required to connect the parking brake actuator to the axle.
A new driveshaft is required. It is a good idea to obtain the driveshaft from the car/truck that you get the trans from. Again depending on vehicle specifics, the final shaft requirements may be that of a longer or shorter unit than the donor, and a new universal joint may be required to mate to the axle.
These guidelines are not particular to my adapter package. Most of these recommendations may be required with many late model transmision swaps.