When I bought my old '63 C10, the harness was a rats nest of old and new wires, cobbled together by what had to have been a Meth Addict. It would periodically kill the truck for no apparent reason and leave me on the side of the road. I knew enough to be able to sort it out and get going again, but it was annoying.
I took a 3 day weekend and ripped every wire out of the truck and built a new harness from scratch. Best thing I ever did. I never had any electrical issues with that truck again.
I highly recommend you take the advice offered and either buy new, or build your own harness. You'll be glad you did.
