Not being there I can't be sure but if the wiring harness is really hacked up then it might be a good idea to think about replacing it all together. There are many posts here about that.
Other than that, if you just need to get the motor running do a google search on ignition circuits is see just what wires go where to get it going. There are several guys here who will be pleased that you are returning the truck to its 'roots', six volts.
Good luck.


Six volt guy living in a twelve volt world