Otherwise, you are correct in your measurements. Inner diameter is 3.25" around the axle housing. Typical replacement is 1/2" stock, but 9/16" is a nice little upgrade. You'll have to re-drill the holes in the top plate (towards the outer edges only, not centered) but, other than that, it's an easy modification.
As for length (measured from the inner I.D. as well), it's up to you to measure compared to what you have now, minus the size of the block, plus the thickness of the add-a-leaf kit. And since you dind't say what block size you have (measured pretty much in the middle usually) or if your springs are original or aftermarket (could make a HUGE difference), it's hard for us to say.
At a guess though, you'll need something in the 8 to 8.5" range. But that's purely speculation. You can always err to the high side, within about two inches or so, and cut off the excess. Heck, you might even find that you can re-use the existing u-bolts if they're in good enough shape. Many of them have enough thread to work clamp down on a smaller load and not run out of thread.
How much thread is left showing below the top plate now?
Paul
ps: And a welcome-aboard from me too!!