Reversed polarity should not make your points overheat. Leaving the ignition on with the motor not running can do it. The one possibility, assuming your battery is truly 6 volt is that the coil has developed a short inside. The damage could have occurred when the 12 v was hooked up.
If this is a US truck, then the original setup would have been positive ground. Canadian GMCs used Chev chassis and they were negative ground.
