At least your reads. Mine goes to full when I fill up and then starts to drop but then gets stuck at 3/4. Occasionally, it will break loose and read correctly until empty. I'm pretty sure mine is a bad spot in the resistor of the sending unit. I've already checked and rechecked every other connection. Gotta pull the tank.......
Your's sounds like a mis adjusted float on the sending unit. Unfortunately, you'll have to pull your tank to fix that as well. When I restored my 70 442, I actually calibrated my sending unit so that I know I have 3 gallons left when it is right on E. Pretty simple really, just make sure it's empty, pour 3 gallons in and bend the float until it reads E or wherever you want 3 gallons to read. You obviously have to put the tank back into position to check each time you make an adjustment.
Someone may have replaced your sending unit with a used one in the past and the used one may have gotten bent while it way laying around waiting to be used again.
