[AZKen]

Your drawing of green is what the 1964 diagram says. So you can hook the dangle up to GND or cap it off. Your choice. In 1963, 1964, 1964, GMC could not make up it's mind about idiot light or gauges. Bulb check and various circuits. It's all the same result. I think you have the info now. It makes no difference if you hook it up or not if you wiring is per factory diagram. If you don't hook it up, it will only take a few miles to see if temp gauge works. I think the idea was to see it spike to hot while cranking, for confirmation. Try it with and without the hookup to see.