Steering All Over the Place at 55mph+

Sounds like you have excessive play in the steering, which is very dangerous. Check toe-in with a tape measure. Easy to do if your tires have straight channels, and a helper to hold the tape measure. You want 1/8" toe-in for RWD, or 1/16" if all parts are new and tight. The fronts rotate back slightly when driving (depending on steering linkage play). You want the fronts exactly parallel when driving. Any toe-out will cause the car to wander on the highway. As the suspension sags over time, the fronts get toe-out, which is likely why people say old cars wander. Can also check by holding a straight-edge along each front tire and sight at the rear, if same track width front-back. Sight should hit ~1" off the rear, so need a helper holding a ruler (1" inward for FWD, which is easier). Only use an alignment shop you trust. Many who worked at one say some techs just nudge the laser sensors until they get a green printout, without adjusting anything on the car.

I agree with going for the most caster you can. Set fwd cam adjuster as far out as possible and aft cam inward. That moves the upper ball-joint as far aft as possible. You want the line thru upper and lower to project to the road in front of the tire contact patch as possible (for shopping-cart effect). The reason radial tires don't work as well is that they are stiffer in driving direction so doesn't deflect that patch as far backwards as with bias-ply. I used Moog offset bushings (don't follow their instructions which are for a pushed-in top mount after a collision). Another issue is where the lower ball-joint sits. You want them as far forward as possible, for more caster. That is affected by the strut rods to the lower radiator support. If the rubber bushings are degraded, or the support bent aft (or weld repair), or the front nut backed off, that could let the ball-joint move aft. I have seen aftermarket adjustable strut rods. There are also aftermarket adjustable tubular upper control arms. Best are the Moog improved strut rod bushings (2-part disks you don't have to squish thru the hole).

You can't also adjust camber, but will probably wind up fine, with the tires slightly leaning in at top (negative camber). Radial tires can deflect in that direction better to accommodate more camber. Factory spec was leaning out slightly, which is strange today since tuners go extreme on neg camber, some even using rounded motorcycle-like tires to match. Rick Ehrenberg of Mopar Action wrote about suggested alignment specs, which you can find here in a search. I think toe-in is most critical for smooth handling, at least after "no play".