Working a stock truck with 6 cyl on a hot day I've often stopped and opened the hood or at least cracked it while going for lunch or whatever. Let that hot air out the top instead of baking in there.
Driving down the road (hood closed...) with air moving the under hood heat didn't seem to be a problem but thats stock.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-