A 1 inch block would not be bad even a 2 but blocks do tend to make spring wrap worse. The problem with the shackles is they may throw off your pinion angle but even that's mute point since your doing a spring under. Your going to have to do new spring purch anyway. If your going big HP do the shackle for sure but otherwise...what ever is cheape just make sure the block is solid STEEL.
www.offroaddesign.com sells a 1 inch block that also lest you move the axle forward and back if your having problems with the wheel not centered in the wheel well.