Another thing to consider regarding foam.
In most cases the foam that is found in the floor of watercraft is poured in place. This in turn, becomes part of the strength and structure of the hull. Using blocks or chunks of foam will not replace this reinforcement. It is what gives so much shock absorbing ability to fiberglass hulled boats. It has a job well beyond buoyancy. Not using poured in foam could cause a very bad event in rough water.

Additionally, many people do not realize that this type of foam will capture water and hold it. This happens especially in boats that sit outside and do not have a good water flow route to the bilge where it is removed. "Hopefully" before it can give the foam a good soaking.