12 Batteries is very doable but, Thats a hell of alot of weight on that rear portion of your frame. I would definetly look into reinforcing at least your front stress point and rear arches all the way back to your bumper.
Thats about what I was looking for to get an idea you know. I have put switches on trucks before but this is my first car. I have a set of 32 inch telescopic cylinders that I am trying to put in the rear. I also was wondering do your pump brackets come off of your battery rack?