Its really simple on these old fords. All you need is a 12 volt test light. If your temp gauge is out also then your IPVR is probably bad, skip to check "c"
1) All test made with KEY ON... test the sender wire at the tank. You should get a dim pulsating light. a) if you get a dim pulsating light then the circuit and the gauge are GOOD.
Sending unit is bad or tank?unit is not grounded.
Run an extra grnd. to sender and retest.
If extra ground does not fix it then remove unit, check float and arm, make sure float is NOT FULL OF GAS. hook up wire and ground unit. moving float arm should make gauge move. If it does NOT. replace sender unit.
b) If you do not get a pulsating light at the tank then check the terminals at back of gauge. You should get a pulsating light at BOTH terminals.
If you get a puls. light at both terminals then wire to the tank is broken.
If you get a pulsating lift at ONE terminal then the gauge is bad.
If you get NO light then the Instrument panel voltage regulator is bad OR not getting power.
c)Locate and test regulator. ( rectangular box on back of instrument cluster) It should have 12 volts CONSTANT power on one terminal and a puls. light on the other. Constant power on one terminal and no power on the other means the reg is BAD....No power at all means the power source is open..
NEVER GROUND ANY WIRE on the gauge circuit as this could burn up a GOOD gauge.....