Ditto, if you have a factory harness and aren't afraid of a few diagrams, and lots of
you can save some cash on wiring and spend it with Marlin instead. I just got my '94 4.3L Vortec on the road about a year ago and the wiring wasn't really that bad. I bought a factory GM manual for my donor vehicle and a factory Toyota Wiring manual for my '86 Toy, added a couple fuses, and connected the dots. One other thing you'll want is the Diagnostic Com port usually located under the dash on the driver side as part of the dash harness, just cut off as much wire as you can and splice the plug wires into the GM computer.
I probably have enough information laying around to create a simple "how to" write up if I only could find the time. Good Luck, If you decide to use the factory harness and have any questions, post up and I'll see if I can help.