I've done the air shocks and shackle thing in the past. Don't think shackles would even fit behind the valance panel, and air shocks, well, I might as well just use a steel pipe. (Had them on my Challenger and hated them.)
But, as was mentioned, getting the right springs with the correct ride height is also an issue, as well as the cost. I've found the Detroit Eaton springs for about $300 with shipping, and the helper springs look be be around $60 to $75. So a big difference in price (as well as installation time.) Really didn't want to replace springs if I didn't have to.
(But considering the cost of gas, I may be spending more time working on the car than driving it!)