These are the issues that I know of when converting from a crash box to a sm420 four speed:
1. bellhousing bolt pattern is different and input shaft bearing retainer diameter is bigger on the sm420. You will need a bellhousing from 48 through the early 50's
2. the mounting holes for the brake/master cylinder are on the bell housing and your replacement bellhousing will not have any. There have been threads in the past about how to make an adapter.
3. The sm420 is about 2" longer than the crash box. This means you will need to shorten the closed torque tube drive shaft or convert to open custom drive shaft. SM420s were made for both open and closed drive shafts.
Good luck, can be done with mostly off the shelf chevy parts but not a weekend project.
