If you have most of the factory stuff in the car and want it to function, a reproduction harness may be easiest. Then do a separate system to run the add-on, race stuff. Make it hot through a separate power source from the battery, and a constant duty solenoid controlled by an ignition ON circuit. Use separate switches and relays for pumps, fans, etc. We've done this for guys who insisted on keeping the factory wiring and having everything work and it work out well. Factory style connections and terminals will make it easier to use OE switches and components instead of cobbling together a mixture of parts.

Or step up, use modern fuse panels and components and do it from scratch. Some guys want to use the OE ignition switch which is fine. Invest in good parts and pieces instead of the junk they sell at the auto parts. I've used Painless and some of the other "universal" kits that the owner already had, but had to modify and make do so much, I just prefer to do it from scratch since everyone has different requirements.