The primary cause of the gas smell is normally the filler neck hose connecting the filler neck to the tank. The neck sticking out of the cab through the grommet is a separate piece, not part of the tank. The connector hose is about 5" long and has two clamps, one on the bottom at the tank and one at the top at the neck. In some cases one of the clamps is missing, but normally after decades of age the hose itself is deteriorated to the point it's split letting fumes into the cab. Gas residue on the tank would tend to confirm this. Unless you have a hole in the tank somewhere, replacing the hose should cure your problem.
At least it should cure it back to as good as original. Gas stink in the cab was always a problem with these trucks even when new, on a full tank and especially in the summer. It's because the cap is a vented style, so to allow air into the tank the cap also allows fumes to escape right next to the window.
