As I've stated before, the Mustang rear floorpan almost certainly has to be unique. But, if you do the job right, the suspension can be common. So you are moving sheetmetal around common suspension mount points.
The S197 was supposed to have
IRS until management made a stupid decision late in the game.
The IRS was virtually identical to the C170 Control Blade, and very similar to what FoA has now. But FNA and FoA couldn't get it together (the reasons are ....complicated -- but they included fuel tank issues..). There are some who would like to have both
IRS and live axle, but that's the worst of both worlds. The
IRS is compromised due to package, and if the volume is low, it's very high priced (the Cobra
IRS was over $800 penalty). The live axle has package issues -- on jounce you have to clear a lot of space in the fuel tank area to allow clearance.
IRS also has issues, including the forward mount point for the CB and weight.
The front structure can be very similar or even common to a sedan as I have mentioned earlier. And you might be able to carry the commonality into the front floorpan (but shortened).
Some of this doesn't matter so much because the metal is stamped in two locations so two sets of tools are required. And the rear of the car requires a lot less development from a safety standpoint so it's less engineering intensive.
But nobody on this board (as far as I am able to determine) can answer any questions on what the Mustang wheelbase should be. To repeat, there is a lot going on in the rear of the Mustang, both above the floorpan and below it. It is by far the most difficult part of the car to engineer, and it is more art than science. The movement of any piece in the rear affects other bits, and you have to keep playing with them until you come up with a solution that works.
What it takes is Ford's Advanced Package guys to sort out a solution and slap some drawings on the wall (yes, yes...it's all done on computer, but there is no substitute for having full size package drawings to demonstrate the proposal).
I'm think we can be very sure Ford already has an excellent grasp of a potential solution set as a result of the GRWD studies.