You're getting anal here. Use 180 grit sandpaper, dry. It will load up like crazy, which is what you want as the swarf will be trapped in the paper. Follow the curve of the cylinder when you clean so no streaks run on a radius. After the block is cleaned of old gasket and corrosion go over it with a rag and solvent and blow out the cylinders with compressed air and clean out the bolt holes thoroughly.