You're on the right track, IMO.
Since it seems we need to scale the "height" of the top template between the vehicle centerline and max camber point/apex, this will appear much smaller than the side application. I believe this is correct. (I'll try to edit the image and re-post).
Yes, it will end in a much more prevalent fin shape in the rear, and I think this is also correct. You would want to continue the natural top-to-side (of the truck) radius along the edge of the boattail. If you change it to a larger radius the angles will be altered.
So, imagine the right form here, with radii on the edges and the sides brought together enough to form that fin.
Updated Patrol templates... hopefully these are correct
Ideal Wagon Form by
Tyler Linner, on Flickr