You have already bought the tank, sorry I missed that. I have two 11 gallon tanks under my trailer. They are 5 inches thick, 22 inches wide and 38 long. The lower the water the better for stability on tough trails. Mine set right above my axle, which helps protect them. On the production trailers, I will have skid plates, but they are 24 inches above ground, so feel safe with them. I think there are pictures of the tanks within the first couple of pictures in my build thread.
If you are stuck with what you have, I would put it in front of the box some how to centralize the water weight. If you have batteries up front, then maybe on the rear somewhere to equalize weight. I need to go back and look at your build again. I watched it develop, but have forgotten the whole set up plan.