So... There is not enough clearance under the rear seat for the battery I got... There is if I take out the sound dampening batting, but then the seat sits on the battery (I suspect you would feel it when going over a bump). So I have changed tactics, and it actually saves me some storage space.
The battery will go behind the rear seat (there seams to be just enough clearance). I just have to remove the rear seat to access it. This also places it nearer to the dc-dc charger which is good.