I made mine out of 1/2" flatbar, the holes are 9/16" and the bolts I used are 9/16" as well, this is the largest bolts you can use (any bigger wouldn't fit in the spring eye bushing sleeves) and they fit the spring eyes perfectly, without any binding whatsoever. Plus 9/16 grade 8 nuts and bolts are readily available and cheap, I got everything for 10$, get 5" long bolts, because the new shackles thicker.
As for tightening, I saw somewhere the torque is supposed to be somewhere around 48-50 ft-lbs, but since I used stover nuts (all metal locknuts). and tightened the bolts until the rubber bushing started squishing, don't overtorque, or else your shackles won't operate properly, they restrict normal movement rates.
Big Red: For the rear I had a special 2" lift leaf pack made for me, it was cheap here but I don't know how expensive it might be where you are. It's a bit more expensive than an add a leaf, but I have the exact same ride I wanted, it ride a brand new superduty, no lean, nice flex, no shaking, the ride is kinda stiff, but that's how I want it. Then I made my shackle so I'd get another 2" of lift. For the front I got a set of drop brackets, and dropped pitman arm.