A real 67 shelby hood is longer than a stock style because of the shelby extended nose section. you can get hoods that have the 67 or probably 68 scoops that fit stock noses. I think you see for 67 style hood scoops because people like the look better.
The 68 Shelby Hood is actually SHORTER than stock, as the nose has a 3" body panel above the grille. The 67 Hood is longer to match the different nose-piece.
and as said above, I have not seen anywhere a Mustang hood with the 68 scoop, just the 67 scoop or that Elenwh*** scoop. Why not be real trick and fint a way to put in the NACA ducts like on the 69-70 Shelbys?