I don't believe floatation inside the hull would do anything to solve your problem. If you wanted to spend a small to mid-sized fortune in changing the boat to solve the problem, then you might try "micro-ballons" which are used by many racing boats to slightly alter trim and sailing characteristics.
A much cheaper and easier way to solve your problem might be to shift things around inside the boat (add an anchor and chain forward, stow heavier items forward; lighter stuff aft, etc.) until you have achieved the trim you desire.
Foam heavier than air, fill voids (air) with foam to add floatation, I'm not seeing it.
Capta is correct in saying that adding foam inside won't alter the lines, in fact it would actually worsen it by adding more weight in the area where you actually want more displacement. Move weight forward .
I might be incorrect in asking about flotation. It might be buoyancy that I'm looking for advice on. I am looking for ways to keep the stern up when it is loaded down with crew, gear, etc.