BD,
I have had success with the following test.
Fuel Gauge Test
Usually when a fuel gauge on these old trucks goes past full and stays it's because the ground is bad. Could simply be a bad connection, or could be a bad sender. May also have a bad wire from the back of the gauge to the sender. One way to test that is to put a test light at the end where it connects to the tank. Disconnect the wire at the tank to test with the light.
If the wire is good, the test light will act as resistor and the fuel gauge should come back towards empty. You could just touch the wire to a ground, but you have to be careful you don't just hold it there or you could burn out the gauge.
Good luck,
Harold
