[robnolimit]

This can be done in about 8 hrs. It's actually easier if the cab/ft/eng.. is in place. The weight of these items helps to stabilize the chassis. If it's a bare chassis, tie it down (if your lucky enough to have chain hooks sunk in the floor) or wieght it down in the center. A pallet loaded with parts works well.
What do you really get? 4" of drop, with no loss of travel or axle clearance. Now, you can add a 2" block, for a 6" drop, and then a 2" spring for a net 8" drop. And, at 8", the suspesion will feel like it has a 2" spring!
What else do you have to do? The panhard rod will need some attention, - but there are plenty of good adjustable ones around. The front trailing arm mounts should be raised. Nock the rivits out, flip the brackets, bolt and weld back on. You need to raise the bed floor 4". Don't chop the center out, unbolt the entire floor and re-fit it back in 4" up.