Hi all...
I have never lifted a superduty, but I have fooled with leaf spring trucks for years...
If you are going to lift the truck with only shackles, the shackles need to be the lenth of stock shackle and then add 2x the lift you want. One inch of longer shackle is gonna lift the truck one half inch. This sometimes makes for a very long shackle, use your best judgement on this. The longer the shackle gets, the more unstable it will be without crossbracing of the shackle it's self. The longer shackle will change your pinion/driveshaft angle, and may or may not affect things, check it. As to the shocks, it's simple to check on your own, as you are doing the lift, or before. Suspend the truck solidly on jackstands by the frame, not on the axle houseing, and let the axle housing droop as low as it can, and unbolt your shocks. See how much further below the shock mount they can go, and this is the ammount of lift you can go to with out over extending the shock. However, this is not to say the axle can't droop further, it could if forced to, but hanging it is a good way to get an idea of how long the shock is/needs to be.
IMO, adding a leaf from another truck would be a good way to get some lift, might stiffen up the ride, but it would be cheaper. New spring packs, might ride nicer, but gonna cost more. I would add the extra leaf, used, from some other truck, one of the longer ones, and maybe some mild shackles also, depending on what I thought it needed.
Rough ride? Last I checked we were driving SuperDutys, big trucks come with a big ride... **grin**
Good luck...
Greg