Re-Wiring 1975 Plymouth Duster

I'd suggest sticking with factory harnesses, just for the fact that everything corresponds to the service manual. That's a huge advantage when a gremlin appears. I have original FSMs for all my cars, but one to consider is the "manual on CD" by Bishko (eBay link--bonus, it's 40% off at the moment). I have one of theirs for my '82 Imperial (that wiring is insane). If I know I'm gonna get filthy, I just print the pages I need and take those out to the garage. If my prints get greasy or wrecked, I just print more.
Year One has the best 100%-factory-style harnesses. They bought the factory connector tooling decades ago. They're made my M&H Electrical Fabricators, who do top-notch work but cannot sell you one (exclusivity contract; Year One owns the tooling). Classic Industries literally buys theirs from Year One. I've used several and they're excellent.
Get on Year One's mailing list; within a week or two you'll get a 30% off discount code, which is absolutely necessary considering the prices--the dash harness for my Challenger was around $600 without it.