There is a guy in MD that will take a GM engine harness and cut it down to just what is needed to run the engine. Jim's Performance Harness.
Harness: 97 Chevy 4.3, Vortec out of a 1500. In these pictures, I have just gotten it back from him. He cut the harness, flashed the computer and tested it all, all for a decent price.
Little fuse block is for everything in the harness. OBD2 connector, fuel pump relay, everything needed to put it in any vehicle and start it up. Its all keyed from the factory, but he does label it to make it that much easier. Makes it a little more tempting to go fuel injection.