It's a bitch. After never being able to seat the gas cap base properly when replacing my fuel filter, I finally gave up and trimmed a bit off the rubber gasket with a scissors (underneath where it doesn't show) and I now install it after the gas cap is on. Works really well and is holding up after more than a year (1997 SS/SP). And by the way, I don't think there should be fuel coming out of any hoses or pipes inside the tank.
Joe