Reposted, was still typing.
Bottom line is: keep the harness and fuse box. Take it off all in tact as possible. All connectors and all do-dad terminals. Then stretch it out on a BIG table/plywwod. Orient it like it was on the truck. Tie it down temporarily to be able to form it and measure lengths. Study what each fuse is for. You will then start to see what you need and what to cut off. You don't need a bulkhead connector necessarily for your old truck. Just another place to corrode. Factory did it on newer trucks for assembly line purposes, to build subassemblies. You will see that there are fewer wires need, as you said, for the old truck. Mount the fuse box above your left foot somewhere, with wires attached, and begin to mock up the routing into your truck. Some put it under seat I think on AD. Use schematic diagrams from both trucks to stay informed. Use the cut off wires in other places if needed. It takes awhile, working with it all, to see the picture. But you will see. It's pretty simple. Just post with any problems/questions. If at some point you don't like the whole idea, take it out and buy a prewired fuse box for a truck like/near your needs. Keep the harness in case you need to splice in a particular connector or special terminal for the 350/350.
You have three main branches on your harness from fuse box. 1. dash, 2. motor & front lights , and 3. rear lights... and a few odd wires like dome and tranny and column. A few direct hot wires with fusible links, as Jerry mentioned. Everything needs to be fused. More than the original diagram may show.
Then we have to discuss ammeter usage.
