I'm wanting to debate this partly, the box used is an 0411 on the later stuff which is the standard LS car box. You'd definitely need a flash though at the least to run it. You're also talking a significant amount of wiring changes, enough I'd probably want to reverse engineer the harness off the LS engine and splice it into the body harness on the truck. They had the gauges controlled by the ECU, so you pretty much need something for that unless you're shoving some Autometers in.
Not sure but the tranny should be the same. The early ones aren't the same, but that's like 92, 93-94, 95 and then 96+ electrically. A 4.3L V6 will use a "V8" style trans with a similar TC (the LS is longer I believe) versus the "4 banger/60 degree/3800" variety I think you're mentioning which has less splines on the input shaft. Supposedly you can use a Canyon/Colorado converter as a cheap upgrade if you go this route, or they made spacers for a 4L80E you can use. 94 and later had the same clutch counts as the V8s, so you should be good there.