Tie a small piece on rope on one end of a piece new or rust free 2 or 3 ft HD log chain and roll and shake the tank around, the chain will loosen the scale and rust.
Inside tank treatment kits available. New metal tank all as much, get plastic no rust ever again. MTS COMPANY, L.C. - Ford Pickup fuel tanks
I am sure pulling and cleaning the sending unit and giving it the once over will help.