No NV4500 will bolt directly to a sm420 bellhousing.
My preference is to use a 1994-1997 dodge 2wd NV4500 for the swap. There are 2 main reasons for using this particular incarnation.
1.) The pilot shaft of the dodge transmission is 1 inch longer than the pilot shaft of the gm transmission. This allows you to use a 1 inch adapter plate between the bellhousing and the transmission. **NOTE** Advance Adapter sells an adapter plate that will work, but it requirs additional maching to go on a sm420 bellhousing. Either the bellhousing will need to be bored out to 5.125 inches, or the adapter plate indexing ring will have to be turned down to 4.686 inches.
2.) The 94-97 dodge 2wd transmission has a physical speedo gear on the tailshaft which drives a pulse generator. The pulse generator can be removed and a standard Mopar speedo drive housing can be installed in it's place with a variety of speedo pinions. The 94-97 2wd version is the only one that allows you to run a mechanical speedometer. **Note** this is a moot point if you are building a 4wd, as the speedo pinion can be placed in the transfer case.
Now there are 2 types of 94-97 dodge NV4500 transmissions. A heavy duty diesel version with a 1.250 10 spline input shaft, and a standard duty version with a 1.125 10 spline pilot shaft. The later 1.125 10 spline matches the gm clutch, so that one is more desireable. The heavy duty version can have the bearing retainer and pilot shaft swapped out without getting into the transmission... adds about $200 for new parts.
The dodge bearing retainer is too fat for a gm throwout bearing. There may be a crossover throwout bearing that works with the dodge bearing retainer and the gm cluch fork. I chose to turn the snout of the bearing retainer down on a lathe to match the diameter of the snout on the sm420. I don'y remember the exact numbers, but I believe it was about .060 needed to be cut. Take your own measurements.
The only other obstacle was the pilot bearing/bushing. The tip of the sm420 is .590 inches. the tip of the dodge nv4500 is .750 inches. I ordered a bronze bushing from AA. The pilot bushing is the only non original clutch component I am using. The pressure plate, disc, and throwout bearing are the same as the gm spec for my truck.
The swap will move your shifter back about 6 inches.
I did the swap on a 1959 gmc 1/2 ton v8. The swap is pretty much the same for a 6 cylinder, however the 1955-1959 gmc 6 cylinder transmissions sit about 4 inches further back as compared with a gmc v8. I believe the gmc v8 sits about 4 inches forward of a chevy engine (V8 or inline 6) in the same years. Moving the shifter back 4 inches will put it pretty close to the bench seat. The shifter handle may have to be bent to clear the seat.
