This is exactly what happened to mine.
I tried an aftermarket sender, and it was hopeless. Everything about it was different -- the shape of the inlet tube, the location of the float arm pivot point, the length and shape of the float arm, the size and configuration of the windings on the resistor board.
Because of the spare tire well intrusion into the tank, the A-body sender windings are NOT supposed to be regular -- there are tighter windings at one end, and looser at the other. The replacement had evenly spaced windings, and therefore could never be correct at all three positions (full, half, empty).
Fortunately, I found someone who gave me their old sender, and I pieced together a good one from the two, plus the sock from the replacement. I couldn't recommend the new part, though I suppose if you were determined you could tweak it to be somewhat satisfactory.
No, you do not need to drop the tank to remove the sender. The seal is no big deal -- it's just a flat rubber gasket -- easy to replace.
OK I'm going to guess what you'll find inside... If its the OEM sender with square box, The sweeping contact in the sender was originally shaped like top half of a bagel. This would keep it in contact with 2 or more of the resistor winding laps. Since we have weaker brass sweeping against nickel chromium wire the contact eventually wears through to become a fish hook shape. Then it snags a winding lap and stays there. If the winding had stayed flat against the board its wound on the snag wouldn't happen. The wire didn't grow in length, the board shrank.
