I just want to add some clarity about upper control arms, because the OP and others reading this thread will benefit from having more information. For 30 years, I've been told that Mustangs can't handle with the rear upper control arms in place. I've seen countless posts/comments about how these cars can't be used for performance driving with all 4 rear control arms on the car, because of quadra-bind. Quadra-bind is a real thing; I'm not denying that. But the issue has been blown out of proportion over the years because it's a great marketing tool, and because many popular mods make our cars handle worse. If your Fox or SN95 Mustang has the original OEM rubber rear upper control arm bushings in place, they are well past their prime. It's going to handle poorly when pushed to the limit. If you put stiff 1- or 2-piece poly bushings in the UCA location, lower the car too much, and install stiffer aftermarket shocks and struts, your car is going to oversteer like crazy on an autocross course. All too often, a maintenance or modification issue gets referred to as a "quadra-bind" issue. This results in cars that have a handling issue most people can drive around being labelled "undriveable" in performance settings.
Why is this important? Because the idea that you need to completely rebuild your rear suspension to even attempt performance driving in a 79-04 Mustang makes it incredibly hard to get people to autocross their Mustangs. And you can't remove the UCAs or install a Torque Arm in some SCCA Categories. Many Mustangs have been autocrossed in Street Category over the years with OEM upper control arms and rubber bushings. In SCCA Categories/classes with less restrictive rules, it is 100% possible to autocross a Fox or SN95 Mustang with all 4 rear control arms in place. I've used aftermarket rear control arms with 3-piece poly bushings and spherical upper differential housing bushings for years, and I've never experienced quadra-bind.
Installing a rear suspension system that removes the UCAs will improve the handling of your Mustang (provided your other mods and adjustments are correct). Unfortunately, that has become the first piece of advice most people get when they say they want to try autocrossing their Mustangs. It was the first piece of advice I got. I was told my whole setup was wrong. While that didn't deter me, I know for a fact advice like that has deterred others.
I'm not trying to dissuade anybody from sharing their experience with their suspension setups. There's a wealth of information on this forum about all sorts of handling mods. I just wanted to balance out the conversation about upper control arms.