Do not waste any time removing the tank until you verify that the top of the sending unit is grounded to a part of the truck which is solidly grounded to the battery.
I hope 3 hints is the charm.
Full reading with ignition on is a textbook symptom for bad ground to sending unit.
Carl


1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission