You can unplug the sender from the harness and test the gauge. On my 69, the plug is next to the trunk latch, driver side, it is the tan wire. With the key on, sender unpluged, the gauge will read full, then using a jumper wire ground the terminal on the harness plug to the body, tan also. The gauge should go to empty with the key still on. If it checks out, then check the sender ground. It is in front of the tank above the rearend, next to the passenger side tank mounting strap, make sure it has good contact, you might want to sand the area to bare metal. If this all looks good, then the sender might be stuck in the up position, I've seen it before. Or the sender is bad. Hope this helps.
PAWN SHOP FIND
RESTORATION IN 1980...
"You can have it all, my emipre of dirt. I will let you down, I will make you hurt"