Hi John
As the vehicles are are very close to being identical I'd suggest using the Chevy wire gauge as shown on the wiring diagram. Given that much/or all the wire used was steel wire when you change over to copper wire it should have no problem carring the load.
I did a lot of checking around when doing the harness with my trucks and have had no problems. I've posted my notes on wiring harness
http://www.canadianmilitarypattern.c...nformation.htm
Cheers Phil