I'm with mntom and 38chevy. If you are thinking about air ride, no way I'd consider all the hassle of swapping and modifying all the components and/or relocate the axle position on the rails. Think drilling dozens of 5/8" holes sideways on heat treated frame rails. Just find a truck in the junkyard with air ride that is good from the cab back, and buy the entire rear frame section. Have them cut it as far forward as possible. Then cut yours farther back. Roll your back half out of the way, and roll the new one in. One splice and you accomplish both the air ride and frame stretch in one relatively uncomplicated project.
As to the driveshaft, you definitely want the advice of a competent truck driveshaft shop. The length of each shaft, and angle of every u-joint/carrier bearing is important. When I was thinking about a frame stretch I drove the truck down to my favorite driveshaft shop and got their opinion and an estimate on building my new shafts. I modified my planned wheelbase a bit to make it easier to build the shafts. They told me 54" max length for each shaft, and all carrier bearings would have to have the height adjusted to get all the angles right, not just add one more shaft on the end with no other changes. The plan was just to drop the truck off there after the stretch and let them put everything in place.
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