[Drbrown]

After fiddling with the gauge, you've gone after the obvious, the send unit. OE senders are getting hard to find. Have you removed the sender unit and held it horizontal while slowly raising and lowering the sender arm while watching gauge ? You can do that sitting in the cab by using some jumper wires. Try lightly spraying the interior parts of the sensor with electronic cleaner. If you get any kind of a reading on the gauge I suspect that at least there's no "break" in the interior sensor.

Otherwise, sitting in the cab with the sender using a pair of jumper wires (ground and sender connection) connected direct to gauge terminals would indicate if you you had loose or faulty wiring between the gauge and tank.

Try substituting the gauge with one known to work. I have a spare stock mid-1940's you could borrow (mail back and forth ?). The gauge internal part may even be interchangeable with yours.

Note: After the "fix" is found/complete, use a gas-compatible sealant on your sender unit gasket (not all are) when installing it.