I would be interested in how you closed off the slotted area. When I bought my '69 Cuda it took me an hour to dig out all the CR@P that had gotten lodged in the drain areas of the fresh air/cowl area. I live in south texas, there is "NO FRESH AIR"...only 95/100 degree air with 95 percent humidity. Use A/C most of the year. All the cowl/vent area is good for is a darn good place for rust to begin. But to answer your question....there are cars like the 57 chevy that had a duct system that came from the front of the car through the inner fender area (in the engine compartment) and into the cowl for ventilation. I have a friend who has one of the systems in his garage. He removed it off a 57 chevy and plans to sell it. But, it's probably too big for the cuda. Either way you could run a vent tube maybe 4" in dia from the grill through the radiator support over the fender (in the engine compartment) directly to the firewall into the cowl area. Just make sure you put a drain system into the duct to keep water from reaching the cowl to minimize rust.. LET'S SEE SOME PICS OF YOUR MODIFICATION!!!
PS- If I were doing this to my Cuda I would get or make a duct that is oval or oblong so that it lays flat against the inner fender. A round duct wouldn't look very sexy!!
Treblig