Word of warning from someone who has been playing with SN95's for 15 years, Dont change the upper rear control arms or even install poly bushings on them.
Reasons = Maximum motorsports Tech
Upper Control Arm Bushings
Whether or not your Mustang is equipped with an MM Panhard Bar, it is very important to have rubber upper control arm bushings. This is one application where the compliance of a rubber bushing is a benefit.
A four-link is over-constrained, a situation that requires the upper arms to physically change length as the suspension moves. Obviously, the metal control arm cannot change length. But its effective length, the distance between the control arm's two pivot points, can change because of the inherent compliance of a rubber bushing. If the ability of the upper control arms to change their effective length is hindered by a noncompliant bushing material, the suspension will bind up, and not move freely. The only way for the suspension to move is for the metal control arm mounting brackets to bend. The resulting restriction in the ability of the rear suspension to freely articulate will cause poor handling; the car will have a tendency to oversteer, and it may do so in a sudden and unpredictable manner.
Retaining the rubber upper control arm bushings is a necessary compromise for acceptable handling. MM has stock replacement upper control arms and axle-end rubber bushings.
I have personally seen broken parts and broken floorpans result from aftermarket upper control arms.
A number of us call the rear suspension "Quadrabind" for a reason.