Not sure if you have it out...but up to 1964 the sending unit worked off the 0-30ohm range and 65 and later went to 0-90ohm range. If still in the tank disconnect the wire to the gauge and check the resistance with an ohm meter. Use the tank for ground, if no reading..establish a good ground on the frame/body and see if it works. If no reading (with gas in the tank)..then you probably have a bad sending unit.
The gauge has 12v with key on, the sending unit supplies the ground in increments.