If you go to the Superlift website and look up the individual part numbers for their lifts you'll notice that the 4" and 6" lifts use the same drop pitman arm, the same radius arm drop brackets, and same shocks. With my 4" lift to go to 6 all I have to do is either buy 6" coils for the front or put a 2" spacer in, but I don't trust spacers all that much so I'd put new coils. I'm putting the shackle reversal hanger in the back this weekend to get rid of the blocks that came on my lift. They are supposed to create 4.5" of lift. Everything I've read says you don't need to lengthen the driveshaft up to a 6" lift. When Superlift TV did a build up on a 199? Bronco they did not put a lengthened driveshaft in with the 6" lift.
Roger