Teflon inserts can help a bit with ride, but probably not enough to make a noticeable difference. My modern truck has plastic pads between the main and second leaf of the front springs. Like JW51 said, mainly for squeak management.
Old springs can wear to the point where they have a ledge at the end of the secondary leaves and when compressed more than normal on a bump can actually hit the edge of that ledge.
If suspect, the best thing to do, besides shackles and bushing replacement, is to replace the springs themselves. I replaced everything suspension related (including springs) on my '51 and it rides just fine.
