Fold down rear seat vs body stiffness

If you do move the wheel wells you will probably have to give up the rear folding seat, unless you want to do major surgery on the seat back and the related components. It may be also be a pain to get the latches that attach to the wheel tubs to line back up so they function properly. You would probably be cutting into that extra bracing in order to move the tubs in, or at least popping the spot welds for the bracing.
I moved the tubs in on a 71 Demon and got 2 1/2 - 3 inches on each side. This is not an exact science since the factory had some leeway when hanging the original sheet metal, and difference of 1/2 to 1/4 inch are not uncommon. All that seam sealer hides a multitude of gaps.
If you are set on doing the mini tub it might be better to remove the latches, narrow and remount the seat top, and then when the tubs and seat are in, remount the latches in the position that will make them line up and work, and that might not necessarily be in their original mounting holes. If you move the latches, your finish panels might not line up in their original holes - may have to open them up.
There's also the new space opened up by adding the metal to widen the tubs that won't be covered by the finish panels.