The easiest way would be to install new arches. This was just done on original car putting it back from a race car to a street car however Bill Hough is no longer making parts so the probably easiest one to get it from would be Gelsco
Hope these help.
last night I put a leaf blower on low and aimed at the side vent. From 3 ft away. There is an amazing amount of air that is ducted to the rear brakes and a decent amount to the little ducts to the engine compartment.
Not done much lately. So I discovered that the rear louver panels have a turned edge. So using a die based off the dims from my louver dies. Made quick work of making the edges turn with some wood and a maple mallet