[rich weyand]

That's very interesting. Before answering his question, I thought to check to make sure the gauges had not changed. Not thoroughly enough, I guess! The aftermarket gauge suppliers list one replacement gauge for 1973-1987, which led me to believe they were the same.

An interesting question now is which sending unit do the aftermarket gauges need? How can they sell one gauge for that range of years?

The question for the OP is, What sending unit did you put in, and does it match the gauge in your truck?

BTW, if you get those resistor values from the local electronics store, you can check your gauge by connecting the sending unit end of the sender wire to ground through each resistor size and see that it matches what Ray is saying. You can also put an ohmmeter on the sending unit to test it against your infrared temp gun readings. That will allow you to see for sure whether the gauge and the sending unit agree on what resistance means what temperature.