Fuel Sending Unit Help!

Hey guys...I have a 71' Dart Swinger. When I bought it the fuel gauge in the dash did not register, about two days after the purchase I filled the tanked hopped in and starter her up and the gauge registered all the way to full. Seeing this I realized or assumed there was nothing wrong with the gauge, it was most likely that the sending unit had issues. So I pitched a new one online and hooked it up before putting it in the tank and it worked, so I went ahead and installed it into the tank and strapped the tank back in; now the damn gauge isn't working again! I'm think maybe a bad ground? Any suggestions would be much appreciated. Anyone know how to test it without dropping the tank again?

Thanx