what are the specs on that cabinet? where'd you get it? that's not a bad deal
i've heard Aluminum oxide is great if you want and etched finish for paint (sand is fine but I wouldn't recommend using a ceramic based media on a diassabled engine). If you want to just clean, walnut or corn cob is good. If you want a matte finish, glass will do.
i typically blast the cases and avoid the valve cover (if i want to polish it). on a disassembled engine, buy new hardware for the majority of the threads (6mmx10). bolt them down until they seal the entire hole. any mating surfaces, e.g. cyl. head chamber face, top of the cylinder jugs, cut a piece of cardboard to the shape and hot glue it to seal it. make it tight and try to avoid that area when blasting.