If you go here you can get the 76 TSM in searchable .pdf. It should be close enough. They also have the 74-76 Parts manual which you should download, too. They are super handy.
Someone makes a new/reproduction harness for these. ZM Jeeps maybe?
In my experience, I find it easiest to fix a factory harness, even one that is butchered as bad as yours. I usually take one circuit at a time and trace it out. Takes a long time but you learn EVERYTHING about how your truck is wired and what is connected to what.
I usually print up a couple copies of the wiring diagram (from the TSM) and just start marking them up as I go. As you verify the harness/circuits, highlight it as done on the diagram. Remove all the extra crap that has been added to your truck that is aftermarket. If there is an aftermarket radio, chuck it. If there is a brake controller, remove it. Etc. Then verify the circuits that need to work even when the key is off (headlights, horn, brake lights, interior lights, etc.). When those all work correctly, move on to the engine. Starter, coil, alt, gauges. When those work, work on anything left over (radio, clock, heater, A/C, etc.). I recognize I just listed about 12 hours of work (or more) but I think it is the best way to fix wiring. I have completely re-wired a vintage car and I have rebuilt harnesses and this seems the most reliable way to be done messing with electrical gremlins.
I also recognize you are going to find ALOT of problems because it sounds like the previous owner(s) worked around bad parts rather than fix them (hence a starter button that cranks without a key). You will find connectors that are totally corroded. When you do, clean or replace them.
Supplies to buy in bulk: Electrical tape (for bundling wires, MUCH better than zip ties), Shrink wrap (to cover terminals as you install/fix them), male and female terminals. Wire labels (I often times use a small label maker if i have to replace a wire with another wire of a different color, I label both sides of it).