I have a 4.3 TBI mounted on my 2.8. I didn't bore the manifold, which would really make a difference. I'll do it when I get a 3.4 installed.

As for what vehicle to take the TBI from, any of the ones you listed will work (as far as I know). The differences lie in some of the finer details. Mine came off of an S10 (I think). It doesn't have the holes to mount the throttle cable bracket. I still made it work, and you can always have it drilled and tapped for the bracket.

I would just get the cheapest one you can find that isn't totally gummed up. Mine took at least 5 hours to clean, used a whole bottle of degreaser, a package of Q tips, and a dremel. It was pretty nasty. Get a rebuild kit for it. It will have all the gaskets you need.

As was said earlier, use your injectors from your 3.1. This comes up every time the topic is raised.
The idea is to add more air, not more fuel.