Since the shock mount is under the spring, and the lowering block is on top of the spring, the shock needed will be the same. All you are doing is moving the axle up.

Since I'm cheap and don't want to pay a ridiculous price for the MP long shocks, I use a 72-80 1/2 ton pickup rear gas shock to get the extended length. It's a bolt on for b-bodies, and an upper bushing change on a-bodies.