It sounds like a lot of guesswork being done here. Remember that cooling is not only the size of your radiator, it has a lot to do with location. You need a negative pressure drop to carry away the hot air. Equal pressure on either side will mean your fan will run all of the time.
AutoSpeed - Undertrays, Spoiler & Bonnet Vents, Part 2
Do some measurements before you mount anything and I would suggest exploring the locations later model Porsches used in the front fenders. Of course, manufacturers sink millions of $$ and manhours to find what works best.