First, a word of caution. Because of the many variables involved in a swap like this there will not be a simple, one size fits all, package. In addition to the different transmissions available, the flathead engines also have some variations in the thickness of and the amount the crankshaft flange extends from the block. I will need some specific details in this regard should you decide to order an adapter.
My adapter will mount the A-904 (3-spd) as well as the A-500 (overdrive) on the flathead 6. The A-500 is available in several configurations (lockup/ non lockup, electronic control, etc) from which you can select. The A-500 was/is used behind the 318 (5.2), 360 (5.9) and a newer version behind the 4.7. The A-500 was first available in 1988 in the Dakota.
The folks at this web site can help ‘by-pass’ some of the electronic stuff:


The usual challenge when mating a late trans to an older engine is the converter balance. All pre-72 Mopar engines are internally balanced, and by 1980 virtually all engines changed to external balance although the 318 still had forged cranks in trucks into the early 80’s. The 400 and 440 had some forged cranks as late as 78 when the B-RB was discontinued. All 360 engines are external balanced.
If you choose to buy an aftermarket converter for whatever trans you select, specify a ‘zero’ balance configuration. If you choose to use an externally balanced converter then you will need to have it rebalanced, or, the flex plate will either need to be weighted to compensate or a section can be removed. The required 8-bolt flexplate, to match the crankshaft bolt pattern, will be listed as for a 426 Hemi application. Even if your crankshaft only has 4 or 6 bolts, the bolt circle, as well as the bolt pattern, are of the same layout as the 426H. The newer 8-bolt pattern (3.7 v-6, 4.7 v-8, and 5.7 v-8) is not the same (smaller bolt circle). Is it also possible to modify the post-1962 6-bolt flexplate that comes with the 3.9 v-6, 5.2 v-8, and 5.9 v-8? Some folks have reported that they have welded up the existing crankshaft bolt holes in the flexplate and redrilled, however, I would suggest caution. This type of welding operation may require a heat treat operation when finished.
Additionally, the 426H flexplate, either oem or aftermarket (B&M #10231) has only one bolt circle for the converter (10″ dia). Compare this to the converter you select; some converters may need to have new mounting lugs installed.
As far as the adapter package is concerned, the flexplate is the only non-stock part that you need to deal with. The starter is used as is and the dust shield requires only very minimal trimming.
Other considerations for swapping the late trans into an early car include, but may not be limited to the following, depending on exact car-engine-trans combination:
• The original car/engine assembly will likely have 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. Rear engine mounts could be fabricated so as to attach to the two bolts that secure the plate to the block on each side of the engine, however, the trans still needs to be supported.
• The late starter requires 12 volts. Refer to the ‘Electrical’ section for information on combining the 12 volt starter in a 6 volt vehicle.
• New shift linkage will be required.
• The transmission will require that the ‘part-throttle shift down linkage’, also called ‘the throttle pressure governor’, be maintained. This is the linkage commonly found between the throttle cable/rod attachment at the carb and the trans.
• You will no longer have a 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 for the early Mopar 1¾” wide leafs are readily available from trailer supply stores 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.
• Your radiator does not likely have a built-in transmission fluid cooler. A good radiator shop can fit one to your radiator. If you choose to use an aftermarket/stand-alone type cooler then a temp gauge is recommended so you can monitor the temp and adjust the size of the cooler if needed.

These guidelines are not particular to my adapter package. All of these will be required of any trans swap.