Just an opinion, but I have found (in model boat building) that spray foam adds a lot of weight. It can be really really messy and the big thing to watch out for is that it expands rapidly and can split things apart or distort their shape when it expands. I would use an inflatable floatation bag in the bow and stern instead. If you do go with a foam you should try a two-part foam available at hobby shops. It is more expensive, but it expands with more air bubbles and therefore weighs less than the "canned foam." It works much like epoxy- you have a resin and an activator. When you pour in the activator it will foam up very rapidly. I would do a test first. You should pour it in a bit at a time and let one batch fully expand before you pour in more. Make darn sure you have extra room for expansion or it will make its own room by distorting your hull.