Here is everything you need to know regarding Coast Guard guidelines for flotation, testing and just about any other question you have. There are tables for specific gravities, engine weights and formulae for computing. Just click on the topic and you're there.
The type of floatation you use is less important than the amount needed. I'm fortunate to have a Cellect factory here in town and my neighbor's son works for them. I can get "defective" pieces from them. If they don't come out perfect they discard them..http://newboatbuilders.com/pages/hin.html