Coolant temp gauge problem

I admit, I am stumped.

You know where the gauges read, I assume, take notes.

Now measure resistance of both of them, the engine when warm. Do the resistances make sense?

Otherwise, it is time to scare up some test resistors and compare on each gauge. They of course I would think read the same, as the do on any other car of the era

If you have a spare tank sender you can use it, set it with the ohmeter where you want according to the tester photo above, and compare to each.

If you have any test equipment such as a generator/ alternator tester, and it has a field rheostat, you can likely use it