Here are your Golden rules you have to follow
Fuel Tanks for Liquid fuels at Ambient Temperature
3. A Fuel tank and associated equipment must not leak.
4. All fuel related components must be securely mounted.
5. All metal fuel tanks must have an earth path to prevent the build up of static
6. A Fuel Tank must be constructed to withstand the environment, forces and vibration it is likely to be subjected to during normal use.
7. A Fuel Tank must not be fouled by moving parts of the vehicle, or mounted
8. Fuel Tanks must not be positioned near a heat source (similar in installation to that of a type approved vehicle), so that a leak could cause a fire hazard.
9. Fuel Tanks must be positioned so as not to be vulnerable to damage from protruding parts or sharp edges in the event of a front or rear impact.
10. The fuel tank must be positioned so that in the event of a leak the fuel could not accumulate or find a path to the passenger compartment. (if fitted in a compartment there must be a drain which does not exit onto a heat source)
11. A Fuel Tank must not be located in, or form part of the passenger compartment, or other compartment integral with it and the bulkhead that separate the passengers from the fuel tank must be free from holes through which any escaped fuel could travel.
12. The fuel tank must be fitted with a vent (to permit release of a pressure build up) which does not exit on to the exhaust or into the vehicle or be situated at such a height that fuel would be likely to leak when the vehicle is driven.
13. Any fuel filler neck or vent must not allow spilt fuel to be able to fall onto the exhaust system.
14. Fuel tank filling points must not be located in the passenger compartment, luggage compartment or within an engine compartment. (Reprieved until April 2010)
15. A fuel filler cap must positively locate to the filler neck and incorporate an adequate sealing arrangement so that a fuel leak is not possible.
16. The fuel filler cap must either be tethered to the vehicle or be of a lockable type which utilises the ignition key of the vehicle and where the key can only be removed when the cap is locked or an automatically opening and
below the filler cap, it is not acceptable for the fuel cap to be closing, non-removable fuel filler cap (Post 05 / 2003 vehicles only)
Other than those 13 restrictions it's a walk in the park, is it your intention to put the tank in the small area at the back of your frame? I would have thought just an aluminium or steel sheet front, back & top would suffice with no floor, however if you intend to bring wiring etc to the back which you will have to do then you may need to panel in one side as well. My only concern is how you are going to get your handbrake & gear change mechanism into the passenger area with compromising your fuel firewalls.
Alan...