I think it would be wise to fix the grounding problem before worrying about the fuel sender, which is located inside the fuel tank. It's quite possible that both the fuel gage and temperature gage are affected by the grounding problem. Your oil pressure gage is mechanical and doesn't need electricity to operate. On many vehicles the fuel and temperature gages have a voltage regulator mounted on the back of the instrument panel that provides a constant voltage to those instruments so they are more accurate, but I'm not sure that your vehicle has one. If I remember the circuit diagrams correctly, it doesn't. Both instruments need a good ground to work properly, however!!
Good luck with it,