http://ecomodder.com/forum/showthrea...tml#post118447
The instruction is ground-plane to ground-plane, top of vehicle to top of template.
I wouldn't over-think it. Have you looked at the fore-aft profile of The Template?
At any usable truncation, the best you can do is produce the existing contour rearward and inward with a boxed cavity or concave bulkhead. I would limit vehicle length to 18 1/2ft based on the length of a parking space.
There are examples of hitch mounts on Ecomodder. Leaving a gap between the box and car body is similar to the separate bed on a pickup truck — a drag inducing feature. It might be better to use a non-reciever hitch and gain additional attachment points at the old taillight locations.