Love it! Great idea! Looks like you're using the space available very well. Something of a fire proof nature to protect the guns sounds like a very good idea. If you're unable to find a safe to fit your space, and you haven't put up dry wall yet... I know there's various "fire proof", "fire wall", and concrete drywall products available. If you can't get a safe in the space, you can enclose the entire space with some kind of fire protecting/damage reducing drywall/sheet rock. Some kind of variation on this might work well for you.
PLEASE post the results! It looks great!