The tank should have a base to rest on and yes it should breath. Once there is fuel in the tank the bottom will be under a lot of pressure. I would worry also about cracking a weld on that tank due to the tightness of the fit and flexing of the hull. Better to just have one made to your specs. It would be easier to have the fuel fill and vent on the outer hull rather then the CC. Imagine having to jump into the boat every time you need fuel up. You will have to take the pump out of the fuel station receiver, rest it somewhere on the boat....jump into the boat.... fill it up.... the pumps dripping fuel and you rest it on the boat again..... jump out of the boat.... When you get old your not going to want to do all that
strick