Replace 1 bushing, or everything?

I went the full PST kit on my Newport ~15 yrs ago. Pricey and not sure all parts were needed. Since then, both strut bushings tore up, even though only driven ~30k miles, so not sure of the quality.

On my Dart, I have just replaced what was needed. Since the control arms came out, I replaced all bushings. The rubber was old and new ones are cheap. I replaced the lower ball joints since they take abuse (one worn out). I kept the upper ball joints and just added new boots, ditto for steering linkage. All joints seemed tight, so why replace? New parts can be worse. I have been buying new suspension parts piecemeal when I find a good price (ebay or rockauto), often genuine Moog. That seems cheaper than the full kits if you can control shipping costs. Certainly get the "improved design" strut bushings, not the old "push thru hole" single piece ones. If your kit has the later, they didn't do their homework. Ditto for the missing important parts in some kits (check asterisks).

Re polyurethane, I think great for strut bushings, boots, tranny mount and such. I wouldn't use for control arm bushings, since it seems quirky to slide rather than twist the rubber as the factory designed. Definitely not for the LCA since it seems the torsion bar could slide back.

I wouldn't fool with boxing the LCA. Maybe needed for dirt track or such, but not a driver. Ditto for connecting frame rails. Maybe needed for hard-core drag racing, but defeats the uni-body concept and adds unneeded weight.