I helped a buddy rewire a 70s single cab with the inline 6 (the PO had cobbled together some combo of multiple years wiring and it was scary). Used a painless kit that was NOT 'plug and play' but was set up with the right circuits, etc. built into the fuse panel, as well as the right wire lengths (most were overly long, a good problem to have) for almost every part of the truck. I have to say that the kit was very good, ie. well labeled, high quality, and understandable for two guys who had no experience wiring a vehicle (I have an engineering degree and background in electrical from high school, but this stuff was new all the same). The best part though was the support painless provided. Prompt to answer the phone, knew their stuff, and were happy to help with every question. Even tried to ensure we talked with the same guy every time to shorten the intro process. We bought extra crimp connectors and wire tape, otherwise the kit had it all. That truck made it from Florida to Minnesota and last I heard was still going strong.
That is my long winded plug for painless. As long as the kit has your vehicle in mind when it is designed, you can do it with the appropriate amount of time. With the quality of their kit I would happily use it as a starting point for wiring any of these old trucks. Never would I do a new vehicle though haha. A total rewire takes a lot of time with our simple trucks. But every night off with some beers and a buddy can make it a good time.
Edit: I noticed guys talking about having to reuse connectors. We had to do that. Did a lot of crimping. Replaced ones that were questionable. Also bothered to look at the painless site to see what they offered for you. Surprised they don't have something listed. But reading again I guess you already mentioned that. I personally would still go to them first, even if it was a more 'universal' harness, if they said it would work. That would be a big undertaking though.