Binkie wrote:
your comment that states, Depending on the final numbers mine will lose about 100# of water and there will be around 50# remaining.

I don't think you will be happy with that solution. You're doing a major job and you should do it to your satisfaction. I would do whatever is necessary to remove ALL of the wet foam, but don't cut through the outside of the hull. It's easier to rebuild the interior.
This is a job that you have never attempted before, and apparently no one else here has either.All of the advice here is only conjecture, though well meaning and some are good ideas. No matter, you will learn as you go. So get started, think ahead of what you are doing, and you will be successful. Its impossible to get all your ducks in a row on a job like this becuase there are so many unknowns. You will figure it out as you go. Over the years I have done lots of projects like that, with that attitude, and they always turn out successful.