The SM420 from different sized trucks should interchange.
The rear covers differ for open/closed drive shafts (parts can be swapped).
The front cover is different from 54 and earlier, but they can also be swapped (I think).
Parking brake and/or drive-shat brake parts can be bolt-on swapped.
If any of this wrong, we will see quick corrections.
http://www.novak-adapt.com/knowledge/transmissions/manual/sm420
