Consider that most all "big rigs" have their batteries mounted in a box on the frame under the truck. There are boxes on the market that have a screw type lift system in them. I'd find a box that was indirectly vented. That fender well has plenty of room for this sort of thing. Check out four wheeling and off road aftermarket folks, for possibilities; search "Battery Box". Use a couple of heavy duty high amperage terminal blocks mounted in the engine compartment for easy access to the leads, and just run the cables directly to them from the box. Just "brainbustin' " here.