Over the winter I put a Spectra EFI in my 69' Coronet and fighting the same issue with their unit. I didn't check the readings when initially installed the tank and upon filling it with 5 gal of gas the needle didn't move past E on the gauge. Should have been near 1/4 on the gauge. I drained the gas and pulled the tank to check and adjust the sender as needed. Word of caution on the pickup/sender unit on the Spectra EFI tank.....go slow and keep your cool. Getting the unit out will try your patience and you may need to keep easily offended people away from you as your vocabulary will match the difficulty. At least it was for me. My kids heard some very interesting word combos during this process.

Once I had the sending unit on the bench I tested it and had to move the float arm and bend the limit tabs to get to the factory specs. Putting the unit back in the tank was probably 2x as difficult as getting it out....again...go slow. With the tank dry I flipped it over to have the float at max full and hooked up some jumpers to the frame and lead wire and hit the key. It read Full on the dash gauge and it would read 1/2 when I moved the arm (with a yard stick from the filler neck) to a line on the unit I marked based on Ohms for "Half Full". All worked fine throughout the range of the unit.

Reinstalled the tank and put a measured 19 gal into the dry tank. Hit the key and 3/4 full on the gauge and then hand to forehead. Even added an extra gallon to make total capacity 20gal and gauge stayed steady at 3/4. So, for now 3/4 is my Full mark until I dare to try and tackle it again.

The tank is grounded well through the tank straps and even has a ground wire from the unit to the frame, so that is covered and not the issue. It may be in the dash or needs to be cycled a few times through the ups & downs of the fuel level in the tank. I only have about 30 miles of driving on the new tank and time will tell with more miles and fill ups.