Is there an inside gasket that goes between the top flange and the quarter panel? What type of fastener do you use to hold secure the unit to the quarter panel? What length should the filler hose be? Thanks!

There is no gasket between the filler neck and body. The filler neck is attached to the body with 3 sheet metal screws. Our filler hose, #9706 is cut to length. You would need to measure yourself, for your specific installation. Normally 12 or less.