This truck has spindles and bags in the front, 2" blocks and notch in the rear and bags...this is on stock style steelies with I believe stock height tires
Here are two that are on 20's with the same setup to give you an idea how low you can go on 20's. The tires are a little larger than the overall stock height but this would give you a better ride and looks nice with the extra "meat" of the tire.
And last, but not least, IvelDesign's sweet 69 longbed he recently sold...in action

Spindles, blocks, and a c-notch...He allowed the pumpkin to slide up a hair into the wood bed so it's a little lower (1" or so) in the rear than you'd get with a stock bedfloor.