What are the negative aspects that would result from sealing shut the air intake grille located between the engine compartment hood and the windshield, in this case on a 1961 Dodge Pick-up truck?
Maybe none if you describe where you will be picking up the intake air instead.
Often this is where air for the HVAC comes from.
I would mean you would no longer be able to get fresh air into the car with out opening a window. Personally I would advise against it.
You could choke to death on the bad air. You don’t want to be dead for life.
Harmless to both car and driver but your fresh air must come from an open window.