The stock rubber bushings (You could always buy new arms). This is only for the rear UCA i'm talking about. They are only 74$.
Here is Maximum's blurb on this:
These Ford rear upper control arms are direct replacements for the 1979-2004 Mustang (non-IRS) control arms. They will restore performance lost to old and deteriorating rubber bushings. These control arms have the stiffest rubber bushings available, and are much stiffer than what came originally on the Fox chassis Mustangs. Although Ford Racing Performance Parts claims their bushings to be "twice as stiff" as stock, our testing has shown that while they are much stiffer than Fox chassis bushings, they are identical to the 1994-04 bushings.
We performed extensive testing of rear upper control arm bushing materials during the development of our Panhard Bar. When retaining the original four-link suspension, the best compromise between resistance to suspension bind, best control of axle position, best ride quality, and least potential damage to the chassis, is to retain rubber bushings in the rear upper control arms. This is especially important when adding a Panhard Bar. The compliance of a rubber-bushed upper control arm is required when a Panhard bar is added to the Mustang four-link suspension.
When installed on a 1979-93 Mustang, these control arms, with their stiffer rubber bushings, help limit axle wind-up and side-to-side motion, but will not damage your Mustang's upper torque boxes as a standard urethane-bushed upper control arm would.
