The blocks on my 93 are tapered. To get new ubolts I would go to a local spring shop, they can make them up. You will need to know the diameter of the bolt and then the length. To get the block, any suspension lift kit company will have them. I would also hunt up a 1975-85 dodge 3/4 ton 4X4 truck, they all came with 4 inch blocks factory. I forget how many of them I have thrown away!
Why do you want to lower the back of your truck?? What are you gonna do with the front?? Are you just wanting to level the truck? Your drive shaft angle might have to be corrected with shims. If you haul loads, and lower the back of your truck then I think maybe the truck will point the headlights into the trees when loaded.
Michael