I think the throttle body is the one to worry about next.
You can get by with the original one, but the truck will run rough.
I bought an old throttle body off of an 89 (I think) GMC jimmy with a 4.3 v6. The bolt pattern is the same if you want to bore your intake manifold to match the bigger holes in the bottom of the 4.3 TB. I didn't have that kind of time, so I took off the top half (injector assembly)of the old 2.8 TB, and bolted on the top half of the 4.3 TB.
Works pretty good. Problem is, I said I was going to bore the intake at a later date, and here I am over a year later and I haven't done it yet. Probably never will.