Wait a minute here. GMC is + ground right? On a Chevy the wire from the points to the coil goes to the - side, but Chev's use a - ground. With a + ground, wouldn't that wire go to the + side of the coil?
By the way, a backward-wired coil would heat up and fail in a very short while. I'm thinking you need to change the wires around on the coil and see how far it will run like that.
