Fuel Gauge and Sending Unit

Actually most cars don't have those ground straps on them anymore and the gauge still works. It should ground through the sending unit lock ring, to the tank to the tank straps and then the to the body just fine. Most likely if you ground that wire and then turn the key on the gauge should peg as sweatybetty said. If that's the case you need a new sending unit. A unit from any 67-76 A body should work.