Paul,
Here's some before and after:
Now that I look at it, there is enough "breathing room" to draw vapors out of there if you were to install a blower. Although it defies convention, I would consider venting to a spot inside as compared to cutting a hole in the hull. I would also consider a smaller vent like a 4" square s/s flush mount.
IMO, I feel the back bulkhead (bottom image) is important to improve hull integrity as there isn't much lateral support in the area.
Let me know what you decide...