You can put little tufts of string along the top of the cowl and if you like giving your GoPro an adventure, down under/inside the engine compartment and on the radiator.
Hood vents aren't always the best if the engine compartment is designed with a baffled flow like a lot of post 80's cars are where the air is designed to exit under the engine into the tunnel/under-pan area. Unless you have the room to box in an Extractor hood feature behind the Radiator but in front of the engine. Ensure the air wants to ONLY go through the radiator, and then exits freely once behind it.
Even with brake ducts, I would definitely enclose the headlight area to prevent disruptive secondary air entrance under the hood. The only air you want ending up behind the radiator is air that just passed through the radiator.
Perhaps put the oil coolers in those ducts and dump them under the car behind the engine, or out the sides behind the front wheel wells?
Beyond that it does become wind tunnel level stuff. Energizing strakes on the K member, Secondary extractor fans, Venturi extractors, etc...Nobody running the correct engine should need to worry about that in Lemons unless they ripped out all their undertray/splash guards.
Mistake By The Lake Racing (MBTL)
88 Thunderbird "THUNDERBIRDS ARE GO!", Ex Astris, Rubigo / Semper Fracti
A&D: 2014 Sebrings at Sebring (NSF), 2014 NJMP2 Jurassic Park (SpeedyCop), 2012 Summit Point J30 (PiNuts)
2018 Route Sucky-Suck Rally Miata, 2019 World Tour Of Texas 64 Newport