Ride problem

Did you recently go to wider tires, or larger wheels with low profile tires? The grooves in the road might be pulling your wheels around and the steering, designed for the compliant old skinny, tall-sidewall tires, is being overwhelmed. Especially if it's original and bushings are worn out.

Otherwise, check out this on bump steer. To compound the problem, roads these days are worse than they were when these cars rolled off the assembly line, and the mods we are making tend to make it harder for the stock steering to keep everything in line.

Bump Steer, bump steering - FAQ, FFR FAQ

could be worn/sloppy bushings

I'd say it's just the way these things are. When my fastback was stock, it was a challenge to keep the thing in one lane. The huge, stock steering wheel, no-feel power steeing, extremely soft springs, worn shocks, tiny front sway bar and flexy chassis all added up to a horrible handling car. But they can all be fixed by swapping to more peformance-oriented parts. Cars have come a looooong way in 40 years, and what used to be acceptable is now bordering on unsafe.

When was the last time it was aligned? Certainly sounds like new shocks are in order. Probably worn bushings also as indicated in the other posts. You did not comment on the age of the steering and suspension components

If you replace them with stock stuff, don't be shocked when it's about the same when it's finished.

If you have the money the best thing you can do is add Rack & Pinion streering, but becareful because everbody seems to be making a kit now days and there are only a few well engineered ones out there. I have used the RRS rack on 2 cars todate, and have it ready to go into a third soon. Randles Rack also makes a great setup but uses some weird bent tie rod ends. I think they work fine, just look weird. TCP also makes a good set up but on the one I saw you got a much inlarged turning radius from stock. After the new rack is in, Just rebuild the stock arms and add new bushings to the sway bar and the car will drive WAY better.

Flexy bushings are a huge part of the vague "feel" of early Mustangs. If you go something stiffer you'll see a small improvement. Then add slightly stiffer springs (front and rear) , drop the A-arms and inch (Shelby drop), add front and rear sway bars and you will be amazed at how much better it'll drive. Stay away from road-race stiff springs if it's primarily a street car and you'll be even happier. I'm not a fan of the rack and pinion kits, because I don't feel they're necessary, provided your stock stuff works fine. I feel the $2000 could be spent in better ways. I noticed a huge difference just by installing a smaller steering wheel. Mine is a 15" Grant wheel, and it's large enough to still easily see the guages, yet small enough to add a bit more feel to the steering.

I believe that anyone who doen't like Rack & Pinion in one of these cars, has never driven one. The only reason I would not have a new R&P would be if you don't want to disturb the originality of the car. With R&P they steer like a new modern car. If you move the wheel in a R&P equiped car the same amount that you currently have to just to keep it staight, you will be in the ditch on either side of the road. It is that much better! And bump streer, what bump steer. It doesn't exist in a properly designed R&P system.
It's the cost that keeps it out of reach of most people

P.S. Stay away from rear sway bars. They can really upset the balance of the car. Just add a 1" front bar. Do a stock rebuild on the A arms, lower the top one 1" use GT springs front and rear and you will have a fine daily driver that handles well and will not have a harsh ride. KYB shocks would round out the package

Looks like we have different ideas about what works and what doesn't, and that's fine. You are right about me having never driven a rack-equipped vintage Mustang. But after seeing how fast non-rack Mustangs go around Laguna Seca a few years ago at the Monterey Hstoric races, I seriously doubt they would have gone one iota faster witha rack. I kept the power steering because to works fine without even one leak, and it'll turn MUCH tighter than my '88 GT with a rack. Anothet thing to consider is, I built my fastback to drive, and drive it I will. I plan to make the HotRod Power Tour next year, in fact. Now I ask you, what happens when that custom rack has a problem (don't say they never fail, everything fails and always at the worst time) in Upper Nowhere, Kansas? Can you buy parts for it at NAPA?
As far as the rear swaybar upsetting the balance, I know that's a common myth, but I say that anyone who doesn't think they help has never driven one. Hmmm, where have I heard that before? Seriously, stock vintage Mustangs have no balance, so how could you upset it? I've used rear sway bars on street and race cars for years , and have yet to have one even come close to being a problem. My wife's '69 Corvette came stock with one, as did my '88 GT and my Suburban, and I while I don't corner hard in the Suburban, a promise you the Corvette and the GT have seen HARD cornering and neither has swapped ends yet. Try one, I promise you'll be happier than without a rear sway bar. I have an adjustable 3/4" bar from Branda's on mine and it made the car such a pleasure to drive. The key in desiging any modifications to any car is to think for yourself, think evevrything through and don't buy anything just because someone says it's the only way to go. There are lots of ways to happiness and even more ways to misery in chassis design.

What exactly is the problem?

The car is mushy?

It wanders at speed?

Before I tossed a bunch of money at the car, I'd get somebody to do an alignment on it and make sure it had 3 degrees of caster, and look the car over for wore out parts.

Almost all of the vintage cars I see that get modern alignments on computerized machines are done to the wrong specs. When these cars were new, they came with bias tires. Radials need more caster, and nobody adjusted the database.

I'd be really selective on who you used for an alignment shop. In fact, I'd go join the local vintage Mustang club to see who they use. Most modern cars are correctly aligned with a simple "set the toe and let it go". The vintage cars need a lot more work, need a technican who understands they need more work, and need an owner who understands that a $200 bill for a 3 hour alignment job isn't too much to pay.

Good practical advice from your fellow stangers here. I would also recommend sub-frame connectors (welded in). If your replacing the a-arms, other front components and plan to have the car for a long time, I suggest putting in roller spring perches while your at it.

One thing I've noticed that rubber deteriorates quickly and maybe your rubber interfaces (rear spring bushings, ect..) could be original like mine was and just putting new stuff in there will make a big difference for not that much coin. I also recommend 4.5 leafs (stock 4 still sags with a load and 5 leafs maybe too harsh for some) in the back as a start and then go through the front as your able.

New cars are designed for rear sway bars old ones are not. They will help if you keep the old or add a soft rear spring. I will conceed on the rear bar for street use. As far as parts for a custom rack is concerned, This is why I did all my home work before buying a RRS system. You see they use a Saganaw rack that you can get parts for at NAPA. Any streering shop will have all the parts you need to rebuild it. It is the only system that is DOT aproved, it has a tighter turning radius than a stock mustang, no bump steer and host of other benifets. It may not go around the race track any faster but the driver will enjoy the ride a lot more.

I do agree that the car may need more caster even 4* is OK. I have 4 dialed into my FB and it tracks perfect