Air shocks are known for breaking shock mounts on both the frame and the axle.
Springs are the "right" way to raise a car, but be forewarned that no matter how you do it, raising the back of a Chevelle WILL CAUSE WHEELHOP if you don't also change the suspension geometry. You will need something to relocate the upper control arm mount on the axle like these:
http://www.jegs.com/i/Edelbrock/350/5213/10002/-1