fuel gauge issue -'68 Barracuda

no changes form stock - original stuff. I'm sure all meant well but other comments are greek to me. Should I see if a wire is off somewhere? All I've done lately is top off the tank and add some lucas engine treatment..does that stuff eat sending units? Ammeter is still in line - will have to see if seems to be trying to work anyway. Temp I have disconnected and ran sending wire to an after market gauge cluster. Could filling it to max have cause sender to stick?