I had problems once with my '69 C500. Turned out to be the temperature sending unit. Along with the laser thermometer, I would try the old fashion boiling/cooking thermometer with the round dial that goes up to at least 400. take off the radiator cap and let her idle until the thermostat opens. Look at the thermometer and then your temp gauge. This could be one test. Also, if your connection on the bulkhead connector is bad/weak, it'll effect your readings. After all, the gauge is reading resistance from the sending unit and if you add more resistance, it could be reading wrong...

If you do get another sending unit, get a Mopar brand. I didn't have any luck with the junk offered over the counter at my local auto part stores.

Just a thought,,