Steering All Over the Place at 55mph+

My $0.02.....I know EXACTLY how you feel. You are describing my avatar car but mine gets bad at speeds above 70+ mph (partway to the 1/8 mile mark). Mine's not fixed yet but I know it is due to having NO positive caster. Lucky to get it to 0deg caster. Feels exactly as you describe - car wants to swerve and swap ends.

Before it goes back to the track, new SPA fully adjustable UCA's I got from Peter Bergman go in. You can bet it will have at least +2deg of caster to start with. I suspect your alignment likely does not have any or at least has insufficient positive caster. These guys on here made me a believer and my car's antics backed them up!! I encourage you to follow these guy's advice!