here's how the gages and sender work.
Thermal-Electric Gauges (Session 227) from the 1966 Master Technician's Service Conference
And there's the filmstrip that goes with that, along with a pdf version of the booklet at
Browse MTSC by Model Year – 1966 – MyMopar
For the sensor you can place it in boiling water and measure the resistance. Then check it at room temperature.
For testing the gages alone, you can apply a very low voltage, low power battery as a quick check, or supply power at 5 to 6 Volts, with a resistor in the return wire. Different resistors will mimic the sender in different conditions.
When working voltage limiter will pulse 0 to 8 volts when supplied power. It must have a return path for the current or the bimetal will not heatup and open.