Shotgun, due to the air ride, your suspension lift options are really limited. I might be wrong here, but there are no kits currently in production for air ride Expys. So, you have to either go custom or toss the air ride setup.
As for a 6" kit, there wasn't ever a true 6" kit made. Fabtech had what they called a 6" lift kit, but it was truely a 5/4" kit. If you can, you can also get a Rocky Mountain kit, but that was a 10" kit. BUt, I have heard some good and bad things about that kit.
I guess I have to ask the question of what you are trying to get out of the truck that requires that much lift? I am running 37's with a 4" lift (97 Expy, non-air ride). Now, I am pushing the limits of what the truck can handle, but that gives you an idea of what can be done.