There are two large problems with the Mustang suspension. First, the stock control arm locations pull the rear tires up off the track into the wheel wells. This reduces initial hook and wastes motion by pulling the rear end up off the track.
This is mostly a LCA angle issue between the rear mounting holes and the frame box holes. It is made worse with tall tires or lowered cars.
The second issue is the rear end twists in reaction to the drive shaft torque, and that lifts the right rear tire even more. The right tire hardly does any work.
You can overcome some of this with adjustable shocks and air bags or while using exceptionally good tires. That might be enough for good tires on a good surface at modest power levels. But as soon as you start making serious power or if you have marginal tires or are running on marginal surfaces you will have to fix the actual geometry and roll problems.
Another serious issue is the Mustang pulls BACK on the upper arms with considerable force. This eventually breaks the spot welds, and pulls the UCA mounting brackets out of the floors. The left upper mount seems to have most of the pressure. Mine had pulled back about an inch. I tried to pull it back into the floor and weld in a plate, but it was too late. All it did was keep cracking the floor. This was with a year or two of 1.3-1.4 60 ft times.
Before I put any big tire on a Fox, I would tie the upper control arm mounts into the floor pan with a plate inside the car under the rear seat. Once the arms start moving out of the floor, it is beyond home garage welding to get them back in place and stable.
This is a video of rear tires pushing down into the track, instead of pulling up into the wheel wells with a reasonably controlled wheelie. This is because I angled the LCA mounting up in the front compared to the rear. Even on non-prepped tracks, unless the track is real greasy, it makes about 1.2 flat 60 ft.
I actually wheelie less after I optimized things. My front tires just barely clear the track, and this is without any wheelie bars or front end travel limiters.