You don't have to buy all the "goodies" to get a car low. We offer many different variations for a wide range of vehicles. Lucky for you the A-body is extremely popular!
Basically what you want is a "Level 1" kit:
http://www.ridetech.com/store/suspen...y-level-1.html
Here's the individual parts:
You can start with our Cool Ride kit; just air springs you put in place of the stock coil springs and shocks:
Front:
http://www.ridetech.com/store/suspen...nt-system.html
Rear:
http://www.ridetech.com/store/suspen...ar-system.html
You can use what you have or rebuild your stock parts and everything will work.
However, you will encounter a few things you may want to consider:
On the front:
-a Cool Ride will place the shock absorber behind the control arms. If you run a wide tire the tire can rub the shock and decrease turning radius (which is why we run the Shockwave on our 66:
http://www.ridetech.com/garage/1966-chevelle/)
-stock control arms may experience ball joint bind in the lowered position (this is why we use our Strong Arms as they have revised geometry for better handling and corrected ball joint angles)
-our 66 also uses taller spindles for increased camber gain and MuscleBar sway bars
On the Rear:
Now, to get that low with 20's on the rear may require some custom work (like PhillipM was saying on ProTouring). You'll likely have to cut and raise the driveshaft tunnel at a minimum. Then you'll need to check where the rear end hits (it'll hit the frame) If it's not low enough and you want to go lower you'll have to C-notch the frame. Then you'll have to check and make sure your control arms have enough clearance. If they don't you'll have to move the trunk floor (or at least parts of it).
Any of the control systems will work for what you want to do:
http://www.ridetech.com/store/muscle...ems-musclecar/
Once you get everything going you can always upgrade at a later date.
Say you like the ride, but you don't like the loss of turning radius. . .next year you can purchase Strong Arms and Shockwaves. Almost everything is interchangeable and scalable. So build what you want and what you can afford, and if you need help let us know!