Neil -
I see what you mean if the roofline is tapered at or near the ideal angle, as it seems to be on the 163D. Maybe the sides of a car (NACA ducts in front of the rear wheels, for example) might be a feasible option for a rear engine vehicle as the sides (plan view) are rarely tapered ideally simply due to practicality of fitting the rear engine, transaxle, and suspension (unless, of course, one goes to a single rear wheel or a very narrow rear track like Dave Cloud's Dolphin)?
I suspect that the intake at the front (stagnation point) would be ideal only if the duct to the rear could be both relatively straight and smooth... since the suspension is outboard and both passenger and engine are inboard, it seems that it would be a challenge to run a duct to the rear without introducing additional drag along the way. Even going down the center tunnel would be non-ideal since the engine block gets in the way at the tail end.
Probably the most ideal would be intake at the stagnation point, through the radiator, and then exhaust through the hood at or near the base of the windshield, but then the space under the hood has no other practical use other than for cooling.
This may not be the right thread for discussing this detail, but I've wondered about the lowest drag approach for cooling since considering the G3F build (
https://www.facebook.com/SmythPerformance).