What does the temp gauge represent?

You could easily "calibrate" your factory gauge in a way that would be fairly meaningful. HOWEVER this "calibration" would depend on a few things:

1--That the wiring harness, connections, etc, in the SYSTEM that is the temp gauge is in good condition

2--That the voltage limiter is stable, IE not a 40 year old mechanical one.

3--That even though the gauge itself is "elderly," that it too is in a condition that can give repeatable readings.

4--Likewise that the sender is in good condition.

All you really need to do is 1 of 2 things.

A---Just use a second gauge or thermometer to "track" readings on the gauge as the engine warms up

B--Remove the sender and using a heater and water, measure ohmic values of the sender at different temperatures. Then you can substitute those readings for the sender into the gauge circuit and notate where the gauge reads at what resistance.

(Somewhere buried in the wonderland of engineering specs is a chart of the thermister temp/ resistance curve of these senders. I don't have it.)

These are the approximate factory values for empty, 1/2, and full readings on the gauge units. Temp, fuel, and oil (if equipped) gauge units are the same electrically

L = 73.7 Ohms (empty)
M = 23.0 Ohms (1/2)
H = 10.2 Ohms (full)