My order of improving would be:
1. Inspect suspension and brakes and replace any parts that are worn. The usual suspects are sway bar end links and bushings, inner and outer tie rods, rubber brake hoses, etc. Often the rubber bushings in the differential mounting ears are dry-rotted or elongated. This is also a good time to eliminate some of the small deflections in the front end by replacing things like the end link, sway bar mount and steering rack bushings with polyurethane. Replacing things like tie rods or bushings will require an alignment, so you may want to wait until the springs and struts are installed so you don't have to do it twice. A good set of specs for a street fox body Mustang is -1.5 degrees of camber, all the caster you can get and 1/16 inch toe-in. The upper strut mount plates under the hood have a series of holes that can be used to set the camber, after you drill out the big pop rivet that secures the plate. Each hole is approximately 1/4 degree of camber. At stock ride height, pre '90 Mustangs like yours will likely max out to -0.5 degree while later Mustangs can go to -1.5 degrees. Be aware that lowering the ride height will increase the negative camber. Caster is important for stability and increases negative camber during cornering. You may also want to consider installing caster/camber plates at the same time you do the struts and springs. Those will allow you to set caster and camber independently and good ones will also preserve your suspension travel with lowering springs.
2. Adjust tire pressures. Modern performance tires are fairly sensitive to tire pressure. Too low and the side walls get squirmy. Too high and the tires will ride on the crown in the middle. In both cases, the tire patch will deform and grip will go down. Assuming you still have 225/60/15 tires, a good place to start is 40 psi front and 36 psi rear with the tires at operating temperature (not cold! tires heat up with use). Take a tire pressure gauge with you, drive the car around then push the car through some corners (I like cloverleaf on and off ramps for this), then pull into a gas station and immediately check your pressures. Autocrossers and road racers will use optical pyrometers to make sure the tire temperatures are even across the width of the tire.
3. Chassis stiffening. I've got a couple of 1987 Mustang GT hatchbacks. One I've owned since new that has a quarter of a million miles on the odometer and another that I picked last year that has 24,000 miles. The high mileage one has Bilsteins and Ford Motorsport 5300B springs, along with a K-member brace, subframe connectors (short ones but welded in), 4 point K-member brace and a Ford Motorsports chassis stiffening kit (essentially some parts from the OEM convertible chassis bracing). I recently installed a bunch of chassis reinforcement bits on the low mileage one including the Stifflers FIT system bracing (and supplemental pieces), a Maximum Motorsports strut tower brace and some bits from a Ford convertible Mustang stiffening kit. The car is still riding around on the original shocks, springs and bushings but the chassis stiffening has transformed the way the car handles, rides and even brakes. With all the reinforcement in, the car is more neutral, way more predictable, easier to throttle steer and rides better. The effect is so dramatic that I'm now considering doing the Griggs style through-the-floor subframe connectors.
4. Increase the damping (shocks and struts). Improved damping is necessary to provide control of the tire contact patches. You'll want to match the dampers to the spring rates but you might be surprised at how well the stock springs work with good dampers like Bilstein or Konis (the red street version).
5. Springs. Yup way down here after you've done all of the above. Unless you've stiffened the flexible fox body chassis, the primary effect of stiffer springs will be to cause the chassis to deflect more, especially if you've increased the grip with sticky tires. The television show "My Classic Car" recently featured a stock 1987 Saleen Mustang running around an autocross course. You could literally see the wheel camber change as the unibody flexed. All the uncontrolled motion caused the car to slide around the course. Do that long enough and the the hood gaps will change and quarter panels will get wavy. At a minimum, do full length weld-on subframe connectors. When picking spring rates, remember that suspension travel is a good thing. It allows the suspension to soaks up bumps and road surface irregularities, instead of bouncing over them. Grip is greatly improved when the tires are actually in contact with the road. Super stiff race springs belong on a race car that is driven at high speed on a relatively smooth track. Lowering the car a modest amount (1"), will help control body roll and weight transfer but lowering the car too far (more than 1.5") will make handling worse, not better, unless you've made some major changes to suspension geometry.
6. Performance tires. You may want to consider switching to larger diameter wheels as the supply of 15" performance rubber is dwindling. If you stick with the OEM 225/60/15, there are only a few choices for suitable V rated performance tires (H rated or lower tires generally have softer sidewalls and poorer response/feel). There are two V-rated tires available in the stock 225/60/15 size that I know of. The first is the Vredestein Sportrac 5 which is a summer performance tire made in Europe (specifically the Netherlands) and has a treadwear rating of 300 (UTQG 300 AA). It's not suitable for below freezing temperatures but have excellent warm weather traction. I keep a second set of wheels with winter tires. The second tire is the Kumho Ecsta LX Platinum which is more of a grand touring all season tire with a 600 treadwear rating (UTQG 600 AA). It's a pretty decent all around tire with longer tread life but less grip. Works okay in colder weather but not isn't as good as my dedicated winter performance tires. PPI has the OEM fitted Goodyear Gatorbacks from the original molds but they are $300 each! There are also some no name Chinese made tires that claim to be V-rated but I would not trust them.
> I plan on changing out the seasoned steering rack and shaft.
Be careful with that. That was one of the biggest mistakes I made with my first fox body Mustang. The 15:1 steering rack is shared with a bunch of other vehicles but the valving is different with most of them having much less feel. When you turn in your core, there's no telling what vehicle your replacement rack came from. They will also often lack the steering limiters needed for the Mustang's wider tires. The one I got had zero feel so I ended up getting a 2003/2003 Cobra rack (requires a hybrid steering shaft available from Maximum Motorsports). With 59K miles, your steering rack should still be fine but you may find the inner tie rods have play in them. If so, get quality replacements (Moog, Motorcraft or similar). If it turns out the rack does need a rebuild, find a local rebuilder that will rebuild your specific core (mark it to be sure).
Dan Jones
