Anyone know this....

[chumbucket]

Shouldn't be a problem as you'll be putting the foam in while the hatch is off. the excess will billow up over the top of the stringers, then when it's cured you can cut off the excess material.

SH - enclosed = key word.
As far as your tank project goes, you will want to secure the tank in position so that it is in place and level when the foam is set. Have fun - the stuff is cool to play with. ;D