Gas gauge problems

Forgot to mention I have checked the grounding strap,it seems to be good,and when I used an alligator jumper from sender to body,the gauge didn't move

Just to be sure... There is a ground strap from engine to body ( firewall) there should be a ground strap from the in tank sending unit metal tube to the fuel line to provide a ground for the in tank sender. at least that's the way it is in 67 may be different in 71