I've replaced OEM Mopar harnesses, re-wired a '53 Ford truck converting it to 12V Neg ground with an alternator (was 6V Pos ground), worked on FJ40/LS conversion, racecars...

If it's an original car/restoration, use a correct repro harness and a FSM.
For hot-rod stuff, I like American Autowire.
If you have the dash out of the car, it's so much easier.