Marc,
The SM 420 is pretty common, it first appeared, (not exactly sure)around the time the AD trucks started in 47 and was in use untill the mid to late 60's. It is a big, heavy almost bullet proof transmission. If you do a search, there is a lot of info on them and check the links section here for Nowak transmissions in Utah. The SM 420 can be found in any number of trucks and junk yards, have great parts availability, and are easy to rebuild. You might even ask the guy you got the engine from if he has one laying around.
Mike
