Nothing makes the "D" impossible. From what I recall of it, the cockpit was lengthened aft of its position on the single-seat models. Place a cockpit in the location where at present the fuselage (and probably reaction mass) exists in the single-seaters and voila! It's a non issue. The "D" in the animation has the same dimensions as the single-seaters do--the cockpit was stretched/stuffed in aft not forward.
The airbrake was eliminated, too, IIRC, so the shield isn't dying for space that I am aware of.