GMCs share many parts with the chevys but some are unique such as the hood and the front fenders on some years. Parts specific for Jimmys are always going to be harder to find. If it is complete there should not much fear though.

Not too familar with the old automatics but it sounds like a hydramatic unit.

The 54 and 55.1 appearance wise are almost identical. Differences in those years are limited to badging and drive shaft design.