Back in the dark ages (1960's) I ran a 47 Mercury convertible with an 11:1 compression flathead V8- - - -and a 6 volt positive ground electrical system. That one taught me a lot about maintaining low resistance connections in the starter circuit. BIG conductor, CLEAN connections, and short ground paths are essential to any 6V system. The best thing you can do for your 6V GMC is to run a #0 or bigger welding cable from the battery (+) post to the starter housing or one of the bell housing to engine block bolts. The flat braided ground strap from the battery to the frame can corrode and induce lots of resistance into the cranking circuit. Run smaller conductors from the engine to the frame and body to ground circuits like lights, heater blower, radio, etc.
Jerry
