I vote for going to restore the rear suspension the way it was designed. If your rear springs are sagging, spend a few extra dollars to replace the rear springs with the correct springs. I've seen several sources for springs at $69-$79 a piece. Throw in good shocks its only 10's of dollars more.
The shock mounts are desinged to take the load of the shocks, not springs and shocks. So even if the spring over shock, or air shocks, do level out the vehicle, you could still tear out your shock mounts and/or body after a few years. Air Shocks, if abused in their use, have a rep of doing this to car bodies, I imagine spring over shocks would easily do the same.
BTW, Air Shocks, used properly, usually work fine and don't damage the body of the car. Proper use of air shocks usually means adding just a little extra spring force to properly working springs, desinged for the load, to level the rear a little, when its properly loaded. Over Load the vehicle, or use the air shocks to make up for bad springs, and they often tear up the shock mounts, body/frame they mount to.