Suspension upgrade?

If all the suspension and steering parts are in good shape I'd start with dialing in the max possible caster (alignment) and doing the "Pump it Down" mod if you have a Saginaw PS pump. Those two things will drastically improve steering feel and tracking at highway speeds. THEN you can start going down the rabbit hole of suspension and steering upgrades lol, that aftermarket for classic Mopars is pretty big these days and still growing.

The very first upgrade I did to my Duster back in 2008 was a Firm Feel Stage 2 PS box when I rebuilt the front end, also used offset UCA bushings for more caster. Before that it was almost undrivable on the freeway, super overboosted (too little effort/feel) steering and I had to constantly correct just to stay in my lane going straight. After that it was great, I drove it all over Colorado it was a fantastic highway cruiser.