Rather than NACA, might consider a "speed mod" described in book "Speed with
Economy" by Kent Paser, Paser Publications, 5672 West Chesnut Ave,
Littleton, Colorado 80123 (1994), Chapter 3, Engine Intake System
Modifications, starting page 52, ". . . use of propellor pressure pulses to
augment the ram air flow into the cowl carb air scoop." . . . ". . .the
scoop was extended forward about 8 inches, and the scoop opening was
rounded-off with an inlet area reduced to about 1.5 times that required for
sufficient flow to the carburetor. . . the scoop clears the propellor by
only 5/8 of an inch, with an inlet area only 10% larger than the venturi
throat area of the carburetor. . . .The scoop inlet is . . .about a 10
degree angle into the advancing propellor blade. . .the sides of this inner
liner diverge at less than an 11 degree total angle.  Any larger . . .and
the scoop would go turbulent"

David Carter