This is right up my alley. I have the full Steeda setup and am now going to Griggs. But it really depends on what you want to do and how you want to spend.
The main problem with most suspension kits is that they don't fix the fundamental handling problems of the FOX (Mustangs since 1979).
The problem with the rear 4 link is that there is inherent bind in the design. Meaning when you start throwing your car around the control arms start working against each other, resulting in the suspension not moving, or binding. This is mostly because the upper control arms are trying to locate the axle both laterly and its roll. To fix the problems you need to replace the entire setup. Steeda and similiar are a band aid solution. But don't get me wrong, with the full Steeda G-Trac III setup and a good set of tires, you can out muscle most other cars. The Steeda 5 link is a very nice and fixes the problems in the stock rear suspension.
The front suspension also has some serious problems. Steeda and others also use a band aide approach.
For some proper setups, check out:
http://www.griggsracing.com http://www.maximummotorsports.com
Both allow yout to do upgrades in steps. I would stay away from KB, if for nothing else because their panhard rod mounts to the spare tire well. The panhard rod is suposed to control the lateral movements of a 3000+ lb car, it needs a little more to mount to then sheet metal. MM and Griggs both mount to the frame.
Anyway, this post is long enough. If you have any questions just ask.
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AIM: Some97Snake