Are you thinking of a tank mounted over the rear axle? Thousands of Cruisers out there running them, Con-Fer among others made them for many years. Most used them for auxillary tanks only but I know of at least a few that used them as primary tanks. There are some issues that would have to be worked through, the extra weight out back and its effect on spring rates, shocks, mounting to keep it out of the suspension travel of the axle and a low capacity for the standard commerically made units.
Newer FJ40's had their OEM tanks under the body in the center, however they had an entirely different body and frame to allow it to be practical, short of some major modifications I just don't see a centrally mounted tank happening under an older FJ40.