Yeh, I have seen a lot of local guys do this. I sold them the engines on many occasions.
It will bolt in and swap over, if you do your homework. Actually, even the drive accessories should bolt from the 4.3L to the V8. Everything should plug in from the original harness to the 5.7. I have installed a 5.7L TBI on a 4.3L engine. I swapped injectors, but only because I didn't want to run the engine to rich. The 5.7 and the 4.3L injectors look the same, if you have the right TBI units (they changed several times a year at one point). I worked at a salvage yard at the time and just went though till I found the right one. If I am not mistaken, the part number for the pump is the same for a 4.3L and a 5.7L, so I don't see it needing any modifications there.
If your wondering, the TBI got smashed in the wreck. It started acting weird about a year after I fixed the truck.
Sounds like a pretty simple straight forward swap to me. Change the motor mounts, find the injectors with the right plugs for your harness, correct fan shroud, and done!
You can also get a vortec 4.3L from a later s-10, and get the TBI vortec intake from 93-94 full-size 4.3L truck. It will all bolt in and be about 40 HP over the stock 4.3L. But, if the engine is already out, why not stick something big in it.