Never underestimate your ability to ESCAPE in a life/death situation. Never underestimate your ability to risk danger for the sake of saving your family. I understand the desire to learn your escape routes and practice, but in the heat of the moment (no pun intended), you will be out of that unit one way or another.
Head first is the way to do it f you have to exit through a port hole. Your gut compresses easier on the bottom. If your gut condenses upward, there's too much body mass up top and you might get stuck. FWIW, I have a business colleague that's about the size of the OP and he goes in and out of 14" and 16" oval shaped port holes on tanks. It's a slow process, but it can be done. The one time he tried to come out feet first, he got stuck and almost passed out due to the fumes.
