70 Dart with factory sway bar ?

Poly bushings do not wear out like rubber. They do not deflect under load like rubber. They do ride a bit harder because of that. Grease them up really good when installing. I still have a 1994 silverado C1500 i bought new. I put polyurethane bushings on the rear leaf springs, and front upper and lower control arms of this truck about 18 years ago when I had it apart for a 4" rear drop and 2" drop front spindle kit to lower it. No deterioration. Everything is still tight and solid. I probably put 100K on it after I did the drop kit and bushings. Not sure this is the answer your looking for. I intend to use poly bushings on both project cars.