I finally decided on location of the rear battery box and divided it up into 2 boxes. Two, because there is a frame that runs directly between the two back wheels and if I want to install the batteries low and cut out the floor pan, I have too keep intact what little frame the unibody car has.
Forward Rear Seat Battery Box Where the floor pan was removed
So I start by cutting out the forward floor for the 4 battery box. I plan on losing about 3 inches of my lower rear seat. Very time consuming. The Dewalt cut off saw is up to the task.

Chop Saw
The box frame is 2″ X 2″ X 1/8″ thick angle iron steel welded together with a Miller MIG. The steel is cut with a Dewalt 14″ disk Chop Saw. This box is 11″ wide, 30″ long and 11.5″ deep. The bottom is 1/2″ plywood. The sides are 1/16″ sheet metal that warps when welded.:-( The top is 1/2 plywood with 2″ flat 1/8″ thick bars.
Battery Box Frame
The forward rear battery box will hold 4, 64 pound batteries. That means all parts of the box including the welds will have to hold 216 pounds while bumping around down the highway.
The next box is the largest and furtherest rear box holding 8 batteries weighing 432 pounds. I used all plywood sides, bottom and top with the 2X2 angle iron corners and 2″ flat center of bottom and top to hold down.

The rear boxes were positioned low; in that the rear box is 4″ above the rear control arm height and the forward rear box only 2″ above this height. The reason I kept the rear box higher is clearance when backing up or angled up driveways etc.