Fuel gauge Intermittently worked,..now nothing. 66 Valiant

You do have a pulse signal going through the fuel gauge and on toward the sender. So the winding in that gauge isn't burned open. Ground the pulsing wire right there and check for needle movement.
Keep these tests brief. Zero resistance will cook a gauge.
When connected you should find the same pulse voltage under the car at the sender. And there have been cases where the wire was broken in the trunk area.
So now I'm wondering about your earlier tests under there? Like where did you ground that sender wire for test. There's a ground jumper over the rubber fuel hose at the sender that sometimes looses connection. Has had come and go connection with weather conditions. Anyway... If that jumper faults, grounding to the sender body wouldn't proof. Lets be absolutely sure which is the fault, sender, gauge, connections, before we pull the panel.