fuel tank
Mario, dont worry about the compressed air tank, that can live anywhere! just remove the pipes and tank and relocate the tank to the most convienient position, then refit the tank and extend the air pipes to suit, its as easy as that, you can buy the pipe from any commercial motor factor for a few quid.
as long as you dont mix up the pipes its not a problem, any valves affixed to the tank can stay on it.
mine was in the same place as yours but I relocated it on the other side, allowing space for the larger tank, the tank that I fitted between the chassis rails behind the gearbox was actually a Land Rover 90 tank that holds 90 L and has a built in pump to transfer, but I am having issues filling it so may change in the future.
If you want a 125L tank let me know I may be able to locate one!