if you haven't resolved this yet, have to verified the resistance to the gauge is changing as the level changes when installed in the tank?

DO BE VERY CAREFUL while doing so if voltage is present on the circuit IE: key on. A friend was trying to adjust the float arm on a 65 corvette while key was on and another watching the gauge and it created a spark and blew the tank. Fortunately it was empty and just fumes but it still took all the skin off his hand and forearm