A few things.

Below is a pick of a battery "box" I did for the '36 roadster. I took a readily available aftermarket base that comes with the "J" hooks and band that goes across the top of the batt. Because of the small size of the batt I use (info later) I cut down the length and then welded on a couple pieces of angle at either end that then is drilled for mounting bolts to secure it to the floor. This is mounted in an area that will be "boxed" off and lidded (hinged) and vented, then covered with carpet.

The battery is an Odyssey, dry cell. I prefer them to the Optima because they are smaller, and, in my and friend's experience, they last longer than the Optima. Being a dry cell they have all of the supposed advantages of the Optima. Size is the other advantage. The footprint is 7" X 6.5", and, depending on what style of cable/clamp you use, they will fit under 6" heigth. I use the PC 925, here are specs; http://www.odysseyfactory.com/specs.htm

As for your car. There are several things to consider. One is having a flat floor. It looks better, and is easier to upholster. I'm not familiar with your body, but there must be some support pieces at the sides to reinforce the upper quarter panels, and the wheel house areas. They are either wood or steel tubing, perhaps a mixture of the two. What I usually do is use thin angle material (typically aluminum has good strenght for thickness), mount it to the side braces as low as practical to make a flat floor and not sacrifice too much space. Leave room for insulation of choice. As you get toward the passenger compartment (going toward the front) you can build in a "step down". This can either be done with the side braces, or you can bond a "brace" to the floor (2x2 or 2x4 lumber, depending on floor contour) For the flooring I use 1/2", high density fibre board (glues and screws together nicely). You can form a pocket for the battery, but still have some form of hold down, and leave enough room to get your hands around the battery for removal.