Actually, you need to have the exit in a negative pressure area if possible. You also need to stop airflow inside the cowling from bypassing the engine. All that requires is that you fill the empty areas beside the engine so there is no space for the air to avoid the engine.

As long as the air must pass close to the engine, then having an exit area that's greater than the intake area will work. You actually don't have to have a massive intake area but a Spitfire wouldn't have that anyway. Just make sure to have at least 1-1/4 exhaust for 1 intake.