A car's nose (in the '60s-'70s) pops the high-pressure air up in an arc that hits the top half of the windshield. That's why there's lower pressure at the cowl. The air closest to the hood is not effective for ram-air; about an inch above the hood is where you start to get into high pressure. There's a drag penalty for too big, too high, too large an opening. Ducting air from the grille area is pretty effective, but it gets heated on its way to the carb, and intake holes & packaging can be a problem.
Look around on the web for the aero test pics on the Mopar Missile Duster, where they tried different scoops.