Sounds like a grounding issue. Is gauge grounded to dash? Is the tank grounded to the frame? The functionality of the installed guage can be tested before final installation. There are other threads on the subject. Have a look and I will see if I can find some pics.

What year and engine do you have? 67 & 68 400's have a return fuel line (vapor line) to the fuel tank.

Attached are some pics of original sending unit.

Here are pics of grounding connects to the tank.