If you want a balls out rear
suspension buy the Griggs, the RRS, or the torque arm from TCI and the non laydown coil spring package.
Rennmachine has used this one (which I think is Heidt's) on a couple of customers cars with good effect. It's not perfect but it's a big step up over stock.
Renn made a comment that, while far from perfect, you could install this and actually enjoy your car rather than argue about what's wrong with it (and from a purist road racing type deal, it's far from perfect).
I've probablly been hanging around Corner Carvers too long, but I'll just say it anyway. These Mustangs are 40 or so year old cars. Technology has changed a little bit since these came out. If you want your Mustang to handle like an M3, find somebody with an M3 and swap him.
I planned to do some OT events with my car, but I've decided now that I'll have too much money into it to wad it up and worse, I know it will be slow. I'm doing the only sane thing I can. I'm building a Spec E30 BMW and run the NASA series, a car that I won't cry about if I wad up.