As for removing the existing tank... one of the reasons I paid to have the tank cleaned out was so that I could work on it safely. I plan to cut up the tank rather than the fiberglass. The gas was pumped out, it's been cleaned pretty thoroughly inside of any and all crud, including scraping out the crystals that were growing on the inside bottom of the tank, hoses have been emptied and plugged at the ends, and the tank has been sprayed with some modern-miracle bacteria that eats gas -- so there are virtually no fumes that we could smell after two hours of ventilating the boat. The tank's been sitting open with both plates off to continue to dry out and the bacteria are, I hope, continuing to feast glutinously. :)

Even with all the steps to eliminate fumes, Nathan, the owner/technician (who was apparently exceedingly well trained and very intelligent, knew his chemistry, etc., had all the latest equipment on his boat, etc.) of MarineLube told me that the recommended way to cut up a tank was to fill it with water, cut through the top in multiple places (letting the water spill into the bilge), and remove it. Perhaps I'll won't have to fill the tank with water to cut it up. If there's no gas smell from the open tank after a week, I see no point... but I'll think long and hard before I put my cutting wheel to it....