Is that 110 octane aircraft fuel? Aircraft fuel is rated differently and it contains some additives to introduce oxygen into the fuel mix for higher altitudes. The true R+M/2 formula we use here in the states isn't used for av-fuel.

I don't see why you can't tap into the stock tank for fuel but I don't know about tapping into the stock fuel pump to feed fuel for both pumps unless you put a boost-a-pump or boost the voltage up to a solid 16v at the pump. That would give you more volume for the in-tank pump.