The '67s and '68s did not have a flapper valve system. I looked at modifying a GTO system but decided it was too much work and yet another opportunity for costly mission creep.

The '67-'68 scoops are not in a negative pressure area, but are too far back on the hood and too low putting them in a low pressure area. This location puts them in the worst part of the boundary layer of air moving across the hood. In other words the relative velocity of the air to the car is much less than the car's speed. For example, at 100 mph the velocity of the air hitting the scoop may be substantially less than 50 mph. Putting the scoops toward the front (ala 69 T/A) or high up above the hood (ala pro-stock racer) overcomes this problem. Facing the scoop rearward near the windshield (ala '70 up T/A) takes advantage of a high pressure zone of air. 1/2 psi can yield a measureable increase in HP so a well designed scoop can be worth it for those wanting to wring out every last HP.

As for the '67 to '68 system, it's just plain cool and that makes it worth adding for some of us.