smooth ride and 8" spring is an oxymoron... but whatever springs ya get, combine it with a shackle flip out back for the first 4.. that'll help tremendously...
and consider a zero rate front and back for another inch, and moving axles..... throw a 1" bodylift in too...
I'm gonna be running 4" spring up front, flip out back, and another 2" for the bl and zero rates on 38 TSL's... tho my front fenders have been cut/sectioned back a bit...
Colbysteven just did 38's and a lift, check his recent thread out or the "sticky'd" "how will my truck look with" thread in the garage...