Several years ago. we had our tanks custom made by All-Rite Holding Tanks
. We sent them the dimensions
of the inside openings of our frame minus 1/2" to maximize the tank capacity without having it drop below the belly pan. We ended up with two gray tanks and a black tank - the black tank sits on the floor with the low-boy toilet on top of it.
The tank install and complete restoration can be seen in our "It's a Girl!!!
" Thread. Your dimensions will vary and I know the prices have changed since 2007.