I don't think it is a big problem. You would probably be fine if you pushed two batteries over toward one side wall, and two over toward the other side wall. I say probably because I'm not there to see how heavy the floor is, etc. The most you would need is to take a piece of angle iron and bolt it across the bottom on the underside. The idea is to distribute the weight.
I don't go with the camp that says your batteries have to be in boxes with vent hoses, but I do believe they should be in some kind of vessel that will catch drips. Just sitting them right on sheet metal is not so good. It will end up being a mess.