1975 Pontiac Firebird Trans Am Fuel Sending Unit?

[troubled31701]

I have a 1975 Pontiac Firebird with a 6.6L 400 in it. The Fuel gauge is acting up and i suspect the Fuel Sending unit is the problem. With what should be a near full (18gallons or so) i am showing off the charts full. I know that this gauge runs on impedance (0-90ohms with 90ohms being full). I am not able to get a reading from the sending unit at all. So i am fairly certain that it is the sending unit...and not the gauge or the wiring leading to the gauge...

Here's where i have run into a problem. For some reason NOBODY has the part i need for my car. Everybody seems to skip 74-75 (not surprising as it was a recession year for the Firebird) I am wanting to know what major differences existed between the 73 and the 75 sending units...and also the 75 and 76 sending units. I am imagining there is only minor differences. I can find those particular years and i am wanting to know if they are compatible with my T/A. If someone else has any ideas on where to find it...please by all means provide me with the link. I have checked with Yearone, National Parts Depot, and Classic Industries as well as local stores. I have found universals, but from what i can tell they are supposed to be used on their fuel cell.
The Tank is grounded properly. The problem is the sending unit....

 

[nicky c]

sounds like you have lost your ground to your tank