What kind of preparation is needed to install a tub surround in my basement?

My basement walls are foam concrete block that have seams every 16" for nailing into and we are putting a bathtub with a tub surround in. Plumbing was prepared for when we laid the basement floor, but we are not sure how to properly put the tub surround up against the wall. Right now we just have a layer of drywall against it-our plan originally didn't include the tub. The longest part of the tub will be back up against the basement wall as well as the short wall that doesn't have the plumbing. Do we need to build in a false wall with 2x4's or can we just put up plywood instead of the drywall to adhere the tub surround? I believe we are just installing a fiberglass tub surround unit.