slugish -
An example of the helper bags Ivan may be referring to may be seen at Ride Rite:
http://www.fsip.com/riderite/products/rrinfo_02.shtml. But, they're mainly designed to level and assist the load capacity of a vehicle. But, you could use them for your application as well. They would help. In fact, with the right air spring, you could reduce a leaf or two from your spring pack and run a setup similar to N2Trux's. But, it's probably a bit more involved then you wanted to go.
Ultimately, you should have at least 3" of travel between the axle and the framerail. Like the others said, you may end up having to c-notch the frame to eliminate the problem.
Another alternative, would be to try hollow air spring/bumb stops. They are self-contained so they don't require any air assistance.With the clearance you mentioned, you may be able to install a single convoluted spring. This way your ride would only get stiffer when the spring compresses, and prevent you from bottoming out. They are available in various load ratings.
