I would not cut the hole. All it will do is let water in. There really is no benefit to any of the ram air systems offered on any of the muscle cars. In all of the testing of the day they did not increase HP by any measurable amount. They are there just for looks.
The 69 Mach hood scoop will not bolt directly onto a 67-68. The 67-68 has a ridge running down the middle of the hood that the 69 did not have. You have to cut a groove in the rear of a 69 scoop to get it to lay flat.
The 68.5 scoop allows for the ridge. There are two reproductions - a $125 one sold by Branda and a $350 one sold by thefordshow on ebay.
The Branda one is very crude. The other is a nice piece.
Also the 68.5 scoop does NOT have the lights in the back and no holes for the lights.