[aerohead]

No easy answer.
The big dogs have access to isobaric profiles at he nose and know exactly where the highest stagnation pressure occurs,which would be where you'd want to harvest all your air from.
Secondly,with so many separate heat exchangers involved,technically,each circuit would be treated individually.I believe that the GM' OLDS AEROTECH of 1987 had eleven discreet and separate 'cooling system' ram ducts and outlets.
*Since Diesels are air-starved,keeping the combustion air inlet wide open is a good call.
*I would attempt to do the same for all other 'systems' except the AC condenser and radiator itself.
*Walter Korff of Lockheed Aircraft Corp. advocated airtight ducts to the heat- exchangers which were at least as 'deep' as the heat-exchanger was 'tall',and just as ' wide'.It's a lot of sheet metal work.NASCAR does it religiously,however they have only the radiator to deal with.
*'Active' systems will have a thermistor located to read the mean average temp inside the system,and the logic portion will vary the signal to a stepper motor which articulates an internal damper to vary the airflow accordingly.Renault used such a system on their Vesta-II of 1987 to help achieve 138 mpg at a steady 62 mph,and 103 mpg at 75.