"Just behind the truck. I can tow doubles, but not planning to do it. Can you describe the e-track set up you have better as far as its locations and length?"
If you have the trailer built for you, they will weld the e-track to the frame and then install the plywood (floors & walls) or you can install it yourself, as far as where to put it, well that depends on what you want to tie-down, as for myself I will be hauling two motorcycles and whatever the wife tells me to haul, so I will be putting complete lengths in, three on the floor (one in the center and one on each side) and two on each wall (one at 18" and one at 48")