Looking at the kitman subframe I notice that it is definately different to the one I have. My kit (356-speedster in the Netherlands) has double tubes at the sill. The lower one is horizontal, just like yours. Then there is one above that, which is angled upwards to the back. The sills of the body rest on this upper tube, and this determines the height of the rear. If your sills are resting on that horizontal tube, I'm not surprised the rear is so low. I would look into body position before changing cooling shroud.
picture for reference