That red wire from starter switch to ballast resistor bottom terminal is to supply full voltage to coil when cranking, it's disconnected when motor starts and then resistor voltage (less than full) runs the ignition. The red wire is on what is known as the starter switch "side terminal". If that part of the switch is bad, it is possible that the starter is taking too much voltage and leaving a weak spark during cranking. Run a direct wire from the big starter stud, provided you clean the rust off of it, to the coil and try starting the truck. This will bypass the "side terminal" on the starter switch to see if it is bad. You will need to disconnect this test wire and turn key off to stop the motor. Make sure all the other stuff you have been messing with is all hooked back up first.
You can also just run a test wire directly from the battery to the coil.
Please confirm your GMC is wired POSITIVE ground and you know that.
