You could try removing the sending unit from the tank, and put longer jumpers from the gauge wiring to the sending unit, and from the sending unit to ground, so you could manually raise and lower the float to see if there is any change in the gauge pointer (if you do this be sure to plug the sending unit hole in the tank to prevent fuel vapor dangers).
Many fuel gauge problems come from a bad ground to the sending unit, if it doesn't have a jumper wire it has to ground from the unit to the tank to body to frame. Also, check to see if there is a possibility the V+ and sending unit wires got crossed on the gauge.
Good luck,
Tim
