Ryan,
As ZoRG as already told you, an open-deck is an engine whose cylinders appear to be 'free floating' in the engine block. This method of manufacture reduces production costs, as it allows the mold to be extracted from the top. This allows the same mold to be used many times, instead of a one time use with a sand-cast mold. Also, in some cases, it produces a lighter engine. Honda makes the most use of this technique, applying it to most of the engines they make. BMW, Daimler-Chrysler, Mazda, Nissan and Subaru also have made use of this method on select engines in their respective line ups.
ZoRG,
Sorry buddy, but I gotta throw my hands up on this one. I've read many papers, and the like, on the subject and am getting too much conflicting information. I've read things that seem to think no less than 4130 chromoly will do, while some suggest that hardened aluminum will do the trick.
Honestly, if gray cast iron fits the bill, but eventually leads to cracking, I would think some grade of a decent steel alloy would do the trick. The problem I see with going with too strong of a steel is getting decent ring wear/seal.
Good Luck
Bryan Carter