A 2wd saff tank will deffo fit in a 3 door mate not sure about the 4x4 one but can't see any reason why not!
As for aftermarket tanks - i bought a new one 2 years ago when building my white 3 door and used it in that for 1500 miles and it now resides in my black 3 door and i have had no problems with it at all
I paid 89 quid for it from a Donno spares day and have seen the same trader selling them for that figure since at Donno!
Cheers Spadge...