I guess I would verify that you had the right fuel sender for your dash first. If you have a digital dash, it uses a different resistance sender than the standard sending unit which is 0-90 ohms. I believe the digital dash was 120 ohms.
At one time racetronix sold a different resistor for the sending unit if you had a digital dash. You had to remove the 0-90 "rheostat" and install the 0-120 as I recall.
If you have the standard unit and the standard analog dash, the trick was to pull the cluster and clean up the post on the back of the fuel gauge where the connector from the sending unit goes. That often fixed the problem.
When the tank is full, you can disconnect the connector to the sending unit at the tank and read the resistance to see if it reads the correct resistance at the tank. Should be either 90 or 120 depending upon the installed sender. If it reads lower than whichever one is low, you can drop the tank, pull the sender and see what the resistance reads when you hold the float all the way up as in a full tank.
That is all that I can think of at the moment
