[spacemanspif]

What you are basically planning is a cowl hood. The proper term for these is "cowl induction hood" as they are used heavily on cars with carburetors because they take advantage of the "bubble" where the bump meets the windshield in order to suck ambient air into the air cleaner. If you want to use it for aero purposes you'd need to seal the cowl to the windshield (or get it very close). I have a 3 inch cowl on my Monte for intake clearance and taped over the vents that allow air back into the engine compartment. If you were to do the "double hood" plan and cut holes in the "under hood", you could run the risk of pulling air INTO the engine compartment, negating the exhaust plan you have. Its my understanding that if you want to remove air from the engine compartment it is best done on the far corners of the hood or even through holes cut in the fenders.

Commercially available cowl hood for Civic:


Buick GNX had some engine heat issues due to the Turbo. Buick added vents on the fenders to exhaust more heat from the engine compartment. It's been said that sitting in traffic with the fan running, the heat coming from those side vents could burn the hair off your arm that is resting on the door when the windows are rolled down.