A really good place to read up on removal of that old tank is
ClassicMako Owner's Club. Those guys know what they are doing.
If the tank won't budge, a lot of time it needs to be "popped" out by jacking alternate corners with a 4x4 piece of lumber / chain setup. Usually it lets go pretty easily once that seal is cracked on the bottom.
Good luck - it's a sh!tty job, but worth it for sure.