Re-arching an original leaf spring will last (keep height) about 20 years. A quality new leaf spring will (also) keep height for about 20 (or so) years. No comment on the cheap Chinese leaf springs (I’ve stayed away from them, so can’t comment about their lifespan. If a person plans on not changing leaf spring capacity (going from 4 to a 5-leaf, or something similar) – I recommend to re-arch your original springs and insert new bushings. If you have an FMJ with a 4-leaf spring or have other needs (high HP racing, towing, etc.) – then a new spring or modification to original spring might be called for. 4-leaf springs are, for most of us, the bare minimum to use (In my opinion), if not under-sprung (is that a word?). Most FMJ vehicles came with 4-leaf springs, except: wagons (most – but not all, have 5-leaf’s), Police/taxi or cars that have a towing package came with 5-leaf's. If you are looking at having new leaf springs made – I would encourage going with the older style round front bushings (instead of oval bushings). That said, most of us will not see a benefit of changing springs just because of the front bushing “type”. The ’66-72 B-body HP leaf springs (5-leaf on one side and 4+2 half leafs on other side) are great for those planning on HP engines – but they are not a direct bolt-on for FMJ vehicles. FMJ total length = 58” (24” front of center bolt / 34” after center bolt). ’66-81 B/R-body = 58” (22” front of center bolt / 36” after center bolt). Mopar Performance Super Stock Spring – 56” (20” front of center bolt / 36” after center bolt). Dakota (not sure of what years) – 56” (24” front of center bolt / 32” after center bolt). I have two old sets of the HP B-body springs (but need re-arched). When I get ready to big block my cars, I’ve considered using the leafs from them to replace the leafs from existing springs (keeping the main leaf in place – but re-arched) – but not done a side to side comparison to make sure that will work, yet. BudW Edit: If you are changing the leaf spring count (increasing the thickness of the leaf spring at the center bolt location), then you will also need to change the lower ISO spring clamp. The lower ISO spring clamp come in 4-leaf and 5-leaf versions (red arrow, below). Personally, I recommend getting rid of the ISO-clamp system and replace with the older style shock plate – for your car ride will be SO MUCH improved. This might be a requirement for some of you – for the 5-leaf lower clamp(s) is not easy to find. Also, the lower clamp is prong to breakage (blue circle) – leaving your car stuck where it broke at. Note: someone added an ill-fitting "add-a-spring", in this case.