To add to Ed Pruss' comment,Ridetech offer a simple bolt-on air bag assist springs which can be added to leaf spring set-ups,here's one rated @ 2000lbs,there are other options available too.

Air over leaf springs.

I think this is what you're looking for.



1950 Chevy Advance Design 3100 in Scotland
In the Stovebolt Gallery
More pix on Flickr.
I've definately got this truck thing in my blood ... my DNA sequence has torque settings
"Of all the small nations of this earth,perhaps only the ancient Greeks surpass the Scots in their contribution to mankind" Winston Churchill.