If you have a F-250 and do a SAS, you need to put the F-350 springs under it. The geometry of the twin I beam suspension uses a lot stiffer spring. If I remember right the 250 springs are almost 3/4 of an inch think per leaf, the 350's are only 1/2 an inch thick. I had the 250 springs under mine for a while and almost couldn't stand the ride. After I put the 350 springs under it ride a lot better, it does still ride like a truck, but better than most Superdutys I have rode in. I did a shackle reversal on mine, just built a crossmember from side to side of the frame with 2x2 1/4 wall box. It was a great place to put a receiver hitch in case I got stuck. The slide in receiver winches work great.
Mine started out as a 2 wheel drive truck also, but I had a donor 4x4 F-250 truck so I had all the same year parts. You would be hard pressed to find a flaw in the 4x4 swap. The shackle flip is the only thing that is not factory. All the lights in the dash are there and prewired if you use the correct transfer case.
Here's a link to some of my pics from a coil sprung SAS I did
http://www.supermotors.net/registry/2272/10542
Below is a pic of my dually and one of the coil SAS I did. Any questions or pics needed let me know.
David