We redid my early 71 using the Painless 18-circuit non-GM universal kit. Came with new blade-style fuse box, some relays, flasher, etc. Worked well with my fuel injection & MSD. I didn't personally install it but helped and observed. The main stock wiring we used was just the set running from door to door for the dome light switch, I think, and another stock connector or two. He spent a little extra time figuring out how to get the headlight motors to work right (using the stock relay), and added a few diodes. We used a non-stock switch for Hazard since the Painless system only runs a couple of wires there.
We lengthened or added a couple of wires, but overall it went pretty smoothly and seems to work very well, less bulky & simpler than stock, better circuit protection & less fire hazard (set up for voltmeter), etc. Would go quicker if carbureted & stock ignition, too. Plus we soldered & heat-shrinked every connection. I was very pleased with that Painless kit.