I have some 52" Superlift 8" rear springs, these are about 7~8 years old but they were always on a pick up (toy only, never used for hauling). Never sagged(?). I even have a set of new Superlift bushings still in the bag.
Let me know if you're interested.
BTW - I recently ordered new 56" springs and the freight was about $30 per spring.