fiberglass rear tub frame Q and fuel tank

[baz-r]

got a bit of a question im thinking of mounting my fuel tank behind the diff/rear support across the back as some other seven type cars (zero) to create an area that can be used as a boot and have a larger tank to have a good range (40l) as i plan to do abit of touring when its finnished.

i need to know how far off the grp frame comes to the back of the tub and how it is affixed to it i have the grp tubes and thair ready to fit.
i was thinking if sitting the tank on top of them and reworking the two d tubes that tie it all in.
would love to hear what everyone thinks and any issues that i havent thort of?

my chassis is at the point where i have all the main bits in up to the rear support fixed to the seat back and im not fussed what type of rear end i have
but i need to start to design and build my tank and still be able to fit it inside of the std back end

i have also thort of using a tank from a std car to save building a tank any ideas what would be a good car too look at
i think someone said mini van tank could be used? anyone?