Gas gauge not working. 1966 Barracuda

Sounds bad, because "I think" a 66 is like the earlier models, IE the gauge regulator/ limiter is inside the fuel gauge. Look and see if it has 3 terminals. It takes 12V at one terminal, regulates for the gauges, internally feeds the fuel gauge, and then sends regulated voltage out to the other gauges on a second terminal. The third terminal goes to the sender.

If the IVR is built into the fuel gauge (3 terminals) and they work, then the gauge may be toast.