73 duster - gas gauge not working

13.5 +/- .5 across the gauge would be correct for the 73 model.
A 10<->80 sender will show about empty and about full in most cases.
It's hardly relevant to how much fuel is actually onboard though.
The earlier 20 ohm gauges will move their needle at 1 ohm increments changes at the sender. There isn't enough heat generated in the later gauges to do that. In fact, engineers changed the needle pivot design to compensate for the weaker movement.
If you know your sender is bad, just ignore all this technical stuff about the gauges for now. Buy a sender and see how it works. Good luck