I think a longbed will ride better than a short bed, for example my grandfather has a 95 GMC 1500 shortbed 4x4. Now you would think a 2500 is stiffer and would ride worse than a 1500, but my dad has a 98 Chevy 2500 4x4 longbed ext. cab that rides like a car compared to my grandfathers shortbed. I think the longer wheelbase really helps the ride.
I've heard body lifts aren't good for nothin, so go with a 6" suspension lift, all springs no blocks would ride the best. Just my 2cents.