If there's an actual rubber gasket, I would assume that the pipe is bolted to the tank. If that's the case, order the new gasket from a Ford dealer, and try crawling undre the car and unbolting the pipe and replacing the gasket.

However, I'm surprised that there would be only a bolted connection instead of a weld, so check that this is the case. Also make sure that there are no cracks or holes in the pipe or around the pipe-tank seam. If thats the case, it'll be more difficult to fix.

 

There is a grommet that seals it up. My Cobra started leaking there badly on day one ( day one for me was last December ). I had the dealer fix it. Cheap part, reasonable labor.

 

Had this problem with my grandfathers ZX. It was a weld in the tank that had rusted through (through lack of use). I cleaned it up with a wire brush, used metal epoxy repair putty and when dry I repainted. Good as new.