I'm about half way through the tanks on mine - I modded the passenger side tank with a fill point the same as the drivers side tank and have the bits to fit a filler in the passenger side identical to the factory one, so it can be filled from outside. I am also thinking of putting a balance pipe in between them but haven't got that far yet. The under seat fill point is just that - pull the seat base and the cover under it and there's a filler in the top of the tank right there. I welded that up on mine when I did the filler mod. A tee valve is the easiest way but again, I'll be using a second pump, solenoid and fuel sender to allow a switch on the dash to change over the fuel between them including the fuel gauge - like in my XJ6 Jag, but that's just me. If you can find a standard auxiliary tank, the filler should be right where you need it already.

