Fuel gauge problem

I’m helping a friend diagnose a problem with the fuel gauge on his 74 Dart. When he fills it up the gauge goes to full but drops like a rock when he drives it. When it hits empty the tank will only hold about 5 gallons and it’s full again and the gauge is back to reading full. We tried a new sending unit and it does the exact same thing. I know the fuel sender for the 60’s cars would read about 10 ohms when full and about 73 ohms when empty and that’s what our sending unit reads. Is the sender the same on the newer A-body’s or did they change the ohmic value? If so my only guess at this point is the fuel gauge in the dash is screwy. Am I missing anything?