What Clyde said:
I have a 91 suburban front crossmember with stock 6 cylinder front springs, 2.5" drop spindles.
rear 2" lowering blocks on rear and 2600 series air bags. have 2 Schrader valves on lisc plate bracket. 1 Schrader is attached to the air tank we installed on the side of the frame along with 2 air control valves hooked to a 3 way switch on the dash of my gmc. this allow us to carry 100psi of air and adjust if we need to.
The 2nd Schrader valve is a backup which allows direct filling of the bags without going thru the valves and air tank.
here is a link to some pictures:
http://kieth.smugmug.com/Trucks/66-GMC/i-HrvtMh3
How my truck sits now after drop spindles and 2" spacer blocks:
http://kieth.smugmug.com/Trucks/66-GMC/i-FZvzkvZ
how it sat before:
http://kieth.smugmug.com/Trucks/66-GMC/i-w9XC84h
picture of air tank mounted to frame:
http://kieth.smugmug.com/Trucks/66-GMC/i-gqt9LcM
picture of spacer blocks:
http://kieth.smugmug.com/Trucks/66-GMC/i-QhxZCfT
tires on rear 275/70/17 wheels 17" x 9" cragars:
http://kieth.smugmug.com/Trucks/66-GMC/i-dJxggwv