The TBI trucks you're looking for will be I think 1988 or 1989 to about 1993. There were some years in the early 90s that offered both the 4.3 TBI and the 4.3 Vortec. The TBI motor can be identified by a large circular air filter on top of the engine, like an older carburated car, while the Vortec (CPI) motors used a square cartridge filter in an housing seperate from the engine.
I've done the v8 swap in these trucks twice, and it's definately not "without a lot of work."
If you're going to look to pick one up, pay attention to any odd front end noises or behaviors. At this age, there are many parts in the front end that could be worn if the truck was used hard. Check things out by driving at slow speeds in a parking lot doing tight turns both ways, note any noises or clunks. Failures I've seen on these trucks are CV joints, lower ball joints, front wheel hubs, and torsion bar mounts. 3 of those 4 parts don't exist on 2 wheel drive trucks.