I *believe* the only major different setups from 1987-1991 for the R/V trucks are:
Small Block
Big Block
Electronic Transmission
All can be made to work one way or another, some require extra parts or work, but they are all BASICALLY the same.
When you get into later stuff, I suspect there are differences like wiring length, ECM type, differnet connectors, fuse panels being different, more or different sensors, etc.
Still probably able to work around, but as many TBI '87-91's there actually are as donors, it's just easier to find what you need off the bat, instead of trying to make something else work, unless the added capability is needed for what you are doing.