blocks go in place of your factory bumpstops (who needs 'em anyway) i use 2" square tube and a peice of flat stock on top with holes and bolts to go thru bumpstop bolt holes with wingnuts for fast easy install/removal. I tack weld the bolts to the flatstock and the square tube rests on the bumpstop perches on the springpack. I have mine raise the back end 1-2" so you actually have to jack the back end up to put them in, then it rests on the blocks. Don't even try to drive on the street that way and go slow through the pits with 'em they will beat you to death!!!
Cheap, simple, very effective VS. expensive bags that will still have some flex/give in 'em. Granted, bags have that nice convenience of hit a switch to air up and another to air down, but if you break one or collapse one its possible it won't be warrantied (we install 'em at work) because they were mostly not made for sled pulling! haha i just don't see 'em workin as good as blocks for pulling. And, bags DO have a weight rating, i'm not sure if sled pulling would really exceed that or not, i kinda think it wouldn't really but i don't know.
Go with blocks! You'll be amazed at the difference!!!