Considering joining the Marines...but I'm not sure I'd qualify?

You may as well apply. The worst that could happen is a rejection, and if you don't apply then you'll always regret not doing it. The spine curve may be the only thing that would hold you back. As for physical fitness, it may be a bit challenging at first but based on some of the overweight Marines I've met, I'm uming it's not quite as physically demanding anymore. A doctor would probably be able to give you a decent idea of your physical limitations, but depending on his/her military knowledge, they may not be able to give you a definite yes/no answer.