Yeah, the tank was built in the boat and to remove the cap I would have to do a whole lot more cutting than making a new bulk head hatch. It has a front hatch on the deck but it's to small and another bulkhead in front of the tank.Are you saying they built the deck over the tank?
It should fit out the hatch.
But nonetheless less, after you open the bulkhead can you cut it in half with a hand saw?
A ground wire to the tank?If you use anything powered, make sure the tank is empty and completely purged out, otherwise we could be hearing about you on the news.
If you decide not to purge it and use an air tool, attach a ground wire to it. Static can and will ignite fuel residue.