[mr48chev]

Had to read the question about three times and I still can't decide if you have an engine that is running hotter than you feel it should or if you have and excess of trapped air under the hood.

To do what you are asking about you would have to block off air flow to the vents and insulate the daylights out of the cab interior side of the plenum for the vents. I'm still not sure if if you will have airflow in the direction you want to draw air out at the bade of the windshield though. It may actually push the air back into the engine compartment at speed or severely disrupt the flow and create a disturbance in he air inside the engine compartment

It's been close around 43 years since I had a TF truck when I had a 57 panel in the mid 70's so I can't remember if there may be a way to vent the air though the back end of the inner fenders and maybe run what amounts to a road draft tube but much larger around and down behind the wheels and into the air flow under the truck. That would create a venturi effect at the end of the tube at speed but wouldn't be much help at low speeds except to let then fan push air out though them.

Or you could do like my grandfather did on his 1 ton and set a stick on the cowl so it propped the back of the hood enough to let hot air out. He did it to get engine compartment air on the windshield to defrost it but it would help air escape.
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