Mine has always leaked at the cap, no matter what cap is on there. Leaks down the filler tube, past the "new" rubber grommet and down in the cab corner area. This is especially true if the tank is full and you go around a corner quickly. I just keep that tank 3/4 full when I fill it up, problem solved.
Also some fuel tank selector valves will leak just enough from the top of the valve to dampen the area around it and give you the smell.

1974 Ford F-100 4x4- 360 / manual.
1970 Ford F250 4x4 ~ Sold.