Steering All Over the Place at 55mph+

Check the tubes where the lower control arms mount in the K-frame, and make sure that the welds aren't busted out.

Also, check the rear suspension. The tail can wag the dog. A loose set of U-bolts, missing leaf spring bushing, or even a bad rear shock can cause what you describe.

You can adjust the play in the steering box, and if it doesn't tighten up any....then you know it's got to get impeached.

In any case, a GOOD alignment shop will catch any of the above maladies.
On second thought, you should probably check for them yourself.

Don't jump into a pile of aftermarket parts to shotgun-solve the problem; there is clearly something wrong. Dress it up after you diagnose it, or you could magnify your problems or worse: Waste money. Even dead stock, with a modern alignment spec, it should handle just fine at triple-digit speeds. I used to think I needed all the race-car gizmos, and then I drove one that was correct and proper, and all of a sudden, it was good enough.