A good 12 volt test light, some patience, and a notebook to write stuff down.
High and low beams, turn signals, park/running lights, a wire to the distributor, maybe three wires to the alternator, a couple of wires to the starter. Oil pressure sender, temperature sender.
Start with a charged battery, and start turning stuff on one thing at a time-- and then find the corresponding wire in the harness with the test light. That will get the easy ones-- maybe only leaving a few to figure out.
That's one way to begin the process.