Hey Dude!
I have used degree shims before and when I installed mine I used C-clamps to keep the spring pack from seporating when I removed the center bolt. I did this with the springs on the truck and the rearend droped out of the way.
Your add-a-leaf should go on the bottom of your pack.
By the way mine were aluminum and they did crack and spit out ~ I found out the hard way = trail carnage. My ubolts came loose and rounded my spring perches. But hey that forced me to weld on new stronger perches at the proper pinion angle.
So even though you are using steel I would re-torque your U-bolts from time to time
