Are the foglights positioned like the picture i have below? That's where the Ford Racing kit has them for my 5.0, and honestly its not a good location. Its too far outboard and not in a high enough pressure zone to really get any air through there, especially at autocross speeds. Honestly, at autocross speeds you shouldn't need ducts at all, but i ended up using these for the road course, and they were exponentially better:
Vorshlag 3" Brake Inlet Cooling Kit for 2013-14 Mustang GT/Boss302 - Vorshlag LLC
I actually have these and a set of Ford Racing baking plates for sale if you really are set on having some though. Shoot me a PM and we can discuss if you'd like.