"Negative pressure" is a little bit of a stretch. Pressure is one of those relative things. You first have to define what you're relating it to.

The hood scoops on both the Firebird and the GTO hoods are in the "boundary layer." The air does not move at the same speed as the air flowing over the car. This is actually a helpful thing as far as pressure is concerned. As far as the pressure distribution on the hood of the car is concerned, there are places on the hood where the pressure is "lower" than others, and in ALL cases (except 1*) it is LOWER than the ambient pressure. (Some dude named Bernoulli proved this about 300 years ago.)

In the example of putting the intake ducts behind the grilles, you'd be adding the stagnation pressure to the ambient pressure and increasing the density of the air (since it is a lot cooler than the air under the hood). This WILL require a re-jetting of the carb, since it will have the same impact as running at much lower altitudes. In the 70's, the early 2nd gen racing T/A had a 2x4 manifold and the shaker scoop faced forward. The dramatic changes in air density entering the carbs made them impossible to tune over the relatively broad speed range the car was racing at. In produciton, the 2x4 was dropped and the scoop was turned around. The reason this worked as Ram Air was because the scoop was above the boundary layer on the hood surface.

There is an area where the cowl meets the winshield where there is a stagnation of the airstream and the pressure approaches the ambient prerssure. The cowl induciton Chevelles capitalize on this fact. It's cold air and its at the highest possible local pressure.

*The one case is when the relative air speed over the surface of the hood is zero, i.e. the car ain't moving.