Oh, and we generally talk about not using blocks, because yes, they add to the leverage on the axle and springs and can lead to more axle-wrap (spring twist) and u-bolt failures.
More axle-wrap puts more stress on the u-joints too, which makes their positioning even more critical.
As for the u-bolts, I would upgrade at least one size anyway. Even without blocks, it can't hurt. With the added stress of blocks, larger is better.
Our Extreme bolts are 5/8" diameter, which is about the largest size you'll commonly come across. A 9/16" bolt is still a nice upgrade and stronger than most people need unless they're really hard on their equipment.
Heck, even a nice new 1/2" u-bolt is a step up from the stock stuff!
And when you pull things apart, inspect the surface of the spring perch for flatness and evenness. If it's uneven, smooth it the best you can. If it's actually warped and not able to accept a block sitting flat, replace it if you can't grind it down flat enough.
They're not that expensive. You just need to be able to weld, or know someone who does.
Paul