I have a Chevy 2500hd. I know that is different than yours but I lowered my truck 1" in the front and 2-1/2" in the rear. The front I turned the torsion bars down till the CV shaft was level/parallel with the ground. At the rear I used lowering shackles.
I am running airbags from airlift that are a custom mount for lowered trucks. They add 2000lbs of capacity. The bags keep my truck from squatting to much and make it ride like a dream when loaded.
I'm not a fan of having a big tall truck. I use my truck and the extra 2-1/2" lower the bed is, makes it a whole lot nicer to load and unload.
On a duelly you could most likely go 3 to 5 inches on the rear alone. They sit really high in the rear. If you do it all with a shackle don't forget to adjust your pinion angle.
Air ride makes a lot of different bags. Call their tech line
2008 Silverado 2500HD DMax LTZ cc sb 4x4 EFILive tuned, Blocked, Deleted, Rerouted and removed B&W Turnover ball and 5th Companion or Pullrite 24K, riderite 5000 airbags
2011 Sabre 32BHOK-6 most of the options