Als,
Where did the fuel tank come from, with the shell or from your donor?
If it came from the donor then you will find a lot of differences as the fuel tanks on a pre 1980 are totally different to a 1980 onwards.
In a 1980 Moke you will find a cylindrical tank that has two metal 'strap' brackets and the filler was actually mounted on the side. The China Moke removes the right angle filler and fills from the top. Probably one of the few improvements I see in the China Moke is the removal of the right angle, but it is about as legal as the rest of what you are doing.
A pre 1980 tank is shaped tank to fit the profile of the side boxes and has a large filler on top. It wouldn't look pretty but it is possible to fit a early tank into a late model shell, however as your shell is not a true copy I do not know what is in the tank space.
Terry