The Classic Trucks harnesses are not necessarily in three separate plugable sections. They are coiled in sections in the picture maybe. If a person is going to rewire a truck, I always recommend going to 12V. It's not any more work as far as harness install and just a little extra to accommodate 12V power. I always recommend a prewired fuse panel harness. There may be separate harnesses sold that can be connectorized to each other, or spliced, that could be done piece meal. That would be a bad idea. When converting to 12V it is better to also include an internally regulated alternator and you can keep the ammeter or install a volt meter. Also not a big deal to do. Your truck, and most trucks here, have very simple wiring. Nothing complicated. You only have two electrical gauges.
To answer your above question. In my scenario, you will rip out and abandon all wiring, fuses, generators..... and install a 12V battery and 10si alternator. It may or may not take more than two days. Lots of variables only you can control about that. A 12 circuit prewired Speedway Motors harness for $189 or equivalent will suffice. With two Classic Trucks part number 55-152.
