If you can locate a copy of Paul Van Valkenburgh's book,Race Car Engineering. He has a section on measuring pressures on the outside of a cars body which would be invaluable,short of a wind tunnel,for siting cooling system exits.
For a 'clean-sheet' race car design you could slect one of the off-the-shelf designs depicted by Hucho or Korff.For a production vehicle you're going to be challenged because of the existing underhood architecture.
Professor Alberto Morelli probably spent millions of dollars perfecting the outlets on the CNR 'banana' car of 1978.
You could not have chosen a more difficult project to take on.
This is truly a case-specific-basis only type proposition.With the variety of vehicles out there,there won't be any one-size-fits-all solution.
