www.gmcpauls.com has some different wiring harness's,not sure if he has what you need. He has an intermediate harness for a 60, but the rear harness is for a 62 and up I think. It can't hurt to look or give him a call.
Joe


54 Triumph TR2(rhd)needs total resto.
60 Apache 10 stepside shortbed
99 Excelsior Henderson