Grym -
Sounds like you may have a fuel tank selector value issue. If it is working properly, you should be able to hear the solenoid inside the selector valve switchover when the fuel selector switch is depressed. Here's a wiring schematic for your '83.
If you are getting a 3 o'clock reading from the fuel gauge, then you have an open on the wiring (pink, pink/white and pink/black) between your gauge, the eletrical connector at the fuel tank selector valve, and the grounding point of the sending unit(s). Check to be sure that the ground wires for the sending units are grounded to the frame (located near the tanks). If the metering switch function of the fuel tank selector valve is also bad then this could also be causing the open connection and the past full reading. BTW, grounding the (pink) wire from the fuel guage should give you a 'E' reading.