I had broken the original springs on my '67 Coronet R/T years ago and when I
replaced them I had the new set re-arched to bring the car back to its
original stance. The car was highly modified and subsequently the springs
were put to the test over several years. The car actually sat too high at
first but after a few weeks the car settled down to the correct height and
seemed to stay that way for all of the 7 years until I sold it. From what I
understand about metals, if you re-arch the springs properly they should
last as long as they did initially from the factory.