How 'bout pulling air in from underneath the Bus. Along with the enlarged NACA ducts, you could use a large scoop and duct from the floor up to the radiator.
Or, with more dificulty, 2 remote radiators placed in areas that see more airflow. Somewhere up front, ahead of the axle if there's room.
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Yetibone
'71 1302S 1.8
'73 1303S 2.3
'83 928S 4.7
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