Yep, seen it done, and if fact did it on a '40 Chevy I had. As mentioned, make a shield to keep it as clean and dry as possible, and plan your mount well so it is strong and accessible. Angle iron works well, and you will want some gussets coming down on an angle from the frame to the outside of the box.

Nice thing is, if you mount it on the same side as the starter you can run very short cables. I would also coat the terminals and ends of the cables with something like liquid electrical tape to put a little more protection from the elements, and maybe use something like a gel battery as there is no way water or dirt can get into those as they are totally sealed and don't sulfate.

Don