I had a similar situation with a 71 Dart Swinger that I had. I remember the springs were wasted and it had some vibration at freeway speeds real low sounding anyways I drove it up to the high desert to pick up the set of 5 or 7 leaf springs off of some Duster. Turned out they were flat when I put them on too, just like the others...lol but I changed the bushings during the swap...it didnt make the drone anymore... and when I got back into to town...i just bought some load helpers. Eventually i bought SS springs . You never know, some characteristics are shared by different issues/parts failing.
