Fuel guage question

[SamHatco]

I just fired up my car this weekend and I've run into a snag regarding the fuel gage. I'm installing an 04 GTO interior in my 69 firebird. The wires leading to the sending unit are as follows:
1. sending unit signal
2. low reference signal
3. 5 volt reference

When I took the sending unit out of the tank, I was able to get the guage to operate by attaching the sending unit to the sending unit signal wire, and the low ref sig and 5v ref wires to the body of the sending unit. When I move the arm from empty to full I got the guage to work. When I installed the sending unit back into the tank the guage would not work. I believe the problem lies with the fact the sending unit is now grounded when in the tank.

Any ideas for a work around or dialogue is greatly apprecitaed.

Sam