[Dead Parrot]

A 3 o'clock reading is generally either a bad ground from tank to frame or broken connection between the gauge and the tank. Open circuit means 3 o'clock.

Some suspects a 41 year old truck are:
Corrosion on the gauge contacts, either between the springs and gauge or between the springs and flex circuit board.
The connector contacts between the wiring harness and gauge cluster.
Bad wire from cluster connector to sender.
Ground from tank to frame.
Dual tank switch or switch contacts if you have dual tank setup.

Quick test of wire at sender is to ground the wire. If gauge shows empty, bad sender or ground. If no change, problem is between that point and the gauge.