Chevrolet should be negative ground, GMC is positive ground.
On a side note, I've seen a 6 volt coil explode from too much voltage. Granted, it had 48 volts
going to it at the time (not my idea). Didn't do the starter any favors either. If you're wondering, the coil was off a '50 Chevy, installed on a '29 Buick Marquette, and was being jumped by 4 12 volt Delco marine batteries. As mentioned, it was not my idea, they guy dong it though "hey, if 12 will turn it over this fast, 48 should really whip it over". I'm not sure which was funnier, the exploding coil, or the melting (yes, melting) starter windings.
