Get some aluminum angle from lowes. 1/8 inch by 1 1/4 should be strong enough. Rivet it to the ribs at desired height. Keep framework level and install your flooring. If you are gonna use wood... use regular non treated wood and seal it first.
Foam is only as good as the floor over top of it. If the floor will sit on the foam itself then use it. But use closed cell foam. Usually pink green or blue. if u are just putting it in to put it in, its only good if the boat flips over. Or takes on large amounts of water. Your boat displaces the same amount of water regardless with or without foam. Weight is the final determining factor in water displacement.
Bilge pump is always a good idea. No matter what the situation.