Fuel Gauge Problem

OK, I give up !! I have had a couple different sending units w/floats replaced, and I've got New Circuit boards behind the Dash ! WTF is wrong with this damn thing ?

I'm working with a '69 Dart GTS 340 Auto. Right now my gauge reads EMPTY. I'd be willing to bet, just as I've done in the past that I go down and fill up the tank I may put 4 gals in it ! Bamm, now it's sitting on FULL ! I've had professionals test the Resistance and found ohms reading the wrong way when full, and vice versa ! So even the Pros aren't able to get it right ! It's driving me nucking futs !!! Previous owner did have a fuel gauge in the cluster of three, and I suggested they check to make sure that it was disconnected so it was causing a grounding problem,not sure if they did that or not though ! Any ideas ??