It's not Great Stuff. It is 4lb density closed cell foam. There is a LOT of difference, but to Joe average it looks the same.
You will need to cut the foam out. You use a pruning saw blade. It is long enough to get to the bottom of the fuel cavity. It takes time.
Then you hook a chain to the tank and use a jack to try and lift one end. It usually will not budge, as there is foam on the bottom side of the tank still gluing it in.
That's when you break out the heavy long bars and with a person on each side at each end you try and rock the tank Port to STB. Then do it on the other side. Be careful when it comes loose that you don't fall out of the boat. Been there done that.
Do not put so much stress on the chain that it starts bending the bracket's on the side of the tank. You don't want to break them off.
DO NOT use ANY electric equipment to cut or vacuum the foam. There is gas in that foam!
I have cut the foam out with a chainsaw before.
If you will look at this link it will give you some ideas. We have done a LOT of R&R tank's on Classic Mako.
http://www.classicmako.com/forum/topic.asp?TOPIC_ID=11506&whichpage=1
Do NOT install the new tank without coating it with Coal Tar Epoxy. This will protect the tank and not let it rot again. DO NOT use GREAT STUFF to reinstall the tank. Get the right stuff, 4lb density 2 part foam. You will not find it in town. fgci.com has it.
If he wants someone experienced with doing this get a hold of me.