Here's where I'm at now. I bought at 15 cubic foot box from K-mart yesterday. Most builds I've been looking at use bigger boxes, but I liked how this one was fairly low. It's still plenty big enough, and should be less like pulling a kite. I'm going to narrow the frame 4" from this pic, and am looking into shortening it.
I'm guessing center the box about 6" in front of the axle? They'll be a cooler mounted on the tongue also. Box is 4' 6" long, in the pics it's 6" foreword.