I believe it is the latter petek. The air entering the radiator will slow creating a high pressure area in front of the radiator. The exhaust side has to re-accelerate the air flow to the velocity of the surrounding air in order to have hope of re-interducing the exhaust air cleanly. The air being heating comes into play as well.
Time for me to dig up Tune to Win again
It just seems as though that amongst others aero issues, the Cobra body just cant get rid of a the air coming in through the nose.
Can't forget the diffusers where needed?
Maybe I should stop thinking about it so damn much and just build the thing!
Do you have the small block tunnel and firewall? If so does it add a lot of room? How is the car coming? Then again, I can just go out and check your site out!