Bump steer kits.

I know this sounds goofy, but before attempting to correct bump steer, make sure it is bump steer. What I mean is, if your car is stock, there is a good chance the problem is the flexy rear suspension rather than the front. When my fastback was stock, there was a corner near my house with a bump in it, and when I hit that bump in my fastback, it would change lanes right now, even though the wheel never moved. From what little circle track experience I have, I knew it was not coming from the front, but rather from the spongy rear springs loading and unloading during the corner. I have found that on early Mustangs, as long as your car isn't too low in the front, bump steer due to geometry problems isn't an issue. How low is too low? If your lower control arm is level or running down towards the spindle, you're fine. Once the spindle end is higher than the pivot on the chassis, you run in to problems.

I bought my rear bar from Branda and it fits great and does wonders for the cornering. I think it was about $80, and it was the adjustable one.

If your car still has the suspension on it defined by your window sticker it is time to put new springs on all around....and probably do a few other things as well.