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Hi Scubadog
One thing that worked well for me was to use a pressure washer. Lots of talk on the forums about cutting the foam with a hand saw but I chose to do it a bit differently. I used a zero degree tip on a pressure washer and it cut through the foam like it wasn't there. Best part is it won't damage the gel coat. I will warn you that it's a messy business but I had the tank cut out in well under an hour. Clean up was probably more than that...
Good Luck