Fuel gauge finally works, but only briefly

Dana67Dart said
I don't know RRR



there is a rehostat in the gas tank that varies the resistance to ground

There is a wire to carry that resistance to the guage

The guage has a heater that changes temperature with a change in the resistance of the rehostat.

That change in temperature bends a bimetal strip that makes the needle move.

The IVR lowers the voltage for the heater in the guage.

Are your gauges on your 64 straight electrical, does the fuel gauge move wildly as the fuel sloshes in the tank?
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No, they are as factory with the IVR inside the fuel gauge. The later models have an external IVR that plugs into the back of the cluster. I am unfamiliar with say, 73 and up, but I suspect they are similar. My 75 F250 has one that bolts to the back of the cluster.