i put some 3400lb SS springs on my coronet a few years ago, haven't driven it much but it's had weight on the tires the whole time.
got them hoping that they would settle like everyone says they do...same arch as when i put them on...
i even have ~100lbs of tools and parts in the trunk and they are still holding...

also, i think there is some good info on the SS springs in the tech archives here. i forget where i read it. maybe a chassis manual or something.