Check out the Mopar Chassis manual if you can get your hands on one (I got a clearance deal off Mancini), there's a whole section devoted to proper functional ram-air hood scoop design. The critical things to get right are the size of the opening (30 sq-in. minimum), distance from top of carb to bottom of scoop, and having the scoop opening raised up off the surface of the hood to bleed off the boundary layer; if you look at the later Challenger T/A or A-body dual-snorkel scoops you can see how the design changed for better airflow.
