I grew up around a shop where we rebuilt untold numbers of Dual Range transmissions. Most of them are pretty interchangeable, with the only major differences being the bellhousing bolt pattern, output housing, etc. If your transmissions are both out of GMC trucks with similar engines, they should swap with no problems. Even the ones used behind the Pontiac V-8's should fit the 6-cylinder engine with just a bellhousing change.
Good luck on finding an experienced rebuilder, as it's been over 40 years since they were in general use. The dual range isn't that hard to build, but it does require a few dedicated disassembly tools. There's a BIG servo return spring in there that must be handled carefully if you don't want it to launch across the shop!
Jerry
