I'll assume the hood latch & hood bumpers are adjusted for best fit & no up/down movement.
Either you limit the air in, or vent it out, usually through the hood. Look at GTLM & GTPro racecars for examples.
I recall a calculation from many years back: it stated that at speed (70+), the air intake opening needed to be only 1/6 the size of the radiator.
Obviously not great at trolling speeds, though.
The valance opening would be pretty effective with an air dam, as many cars (example: 80s-90s Camaros) have proven.
To accomplish the OP's goal, dead-stock appearance is pretty much out the window.