Here's a Fan & Shroud assembly that Wade (MarkIV/4280) fabbed up. The flappers over the holes let air through once you hit a reasonable road speed, and the flappers are sucked closed when the main (ducted) fan is running without "ambient airflow".
He did a nice job on this.
A good idea for you to consider.
Rocky
PS> (THIS IS AN AMUSING ANECDOTE - NOT A SERIOUS PROPOSAL)Did you ever see that radiator switch that had the metal paddle? When the car was moving, the paddle blew back and hit the limit switch - turning off one of the fans. It was on this board somewhere!
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