OK, I'll try to keep it short.
Mustang's rear 4 link design from the eighties to 04 has bad geometry on the upper CA's pivot.
Upper Control arms are supposed to stop axle wind up, and control the side to side movement of the axle.
but the pivot geometry isn't true, so they bind up. So Ford made them have soft busihings and some side-to-side slop.
This keeps the bind down but you get axle deflection and launching traction suffers.
If you change to a stiffer bushing, the bind get's worse.
ever heare of guys warping the torque box??
that's a direct result of stiff bushings in the Upper Control arms. They bind up and transfer the torque into the sheetmetal of the car.
Ford Racing uppers are the only other choice that doesn't make the bind worse.
Better lowers, with stiff bushings and the stock or ford racing uppers is the best route for doing control arms.
Beyond that you're looking at a Panhard which will keep the axle centered so you loose the axle deflection.
Best way is to loose them and go with a torque arm, changing to a 3 link, no bind and superior handeling and traction.
I've done a bunch of research and talking with MM to find this information out.
Manufacturers of Copy Cat arms won't tell you, if they even know, becuase the wouldnt sell their product.
Trust me, AND call Maximum Motorsports about any suspension questions you have.
they have been very helpfull with me, and they actually engineer and test they're parts!!!
their web site also has a good description of what the parts will do for you, and how they're built. I REALLY like that.
Long Live MM