Nobody has written a detailed account of how to modify a stock harness on this website, that I can recall. If you go to LS1TECH.COM and look in their conversion/hybrid section there are a few stickies with good details on modified car harnesses. As for the old vacuum style cruise control, I haven't a clue. Surely it can't be too hard if you find a wiring diagram for how it works in stock configuration. You can probably modify a newer style electronic cruise control module that was mounted on the firewall.