I had a really stuck tank on a 24 Grady I was working on back in '99, All foam was cut away, 4X4 over the top of the gunwhales with a come-a- long hooked to the tank to try to get it to pop. Hurricane Irene came through here and dumped 6" of rain in the tank coffin and the hydraulic effect of the water pushing up on that tank popped it out like a cork. try the come-a-long strapped to a cross member a top the gunwhales and fill the tank coffin with water. She'll pop. One time I had a flywheel on a 28 Johnson that was so stuck that it would not pop so I left the puller on the flywheel and every day about an 1/8 of turn and she popped on day 6. A little time and patience proved worthy.
Disclaimer - I have never tried this. Just an idea.Have you ever seen or used a wire saw? Basically a long flexible abrasive wire with loop handles at each end that you can use to cut trees, etc. in a pinch. If you got a long one, I imagine you could get it started at one end of the tank (wrapped around the bottom), and work your way along the tank, working the wire from side to side, and cut the foam right below bottom of tank. Shouldn't really do any damage to the aluminum.