What do you think was the determining factor for installing the hood seal then if both small and big blocks had them--but not all small blocks?
Have not found any original unrestored SB with them in 68 any plants though you can find a fair number of repaired/"restored" cars with them added by owners in the thought that it would help with the efficiently of the car/cooling. Guess that is what you have been seeing at some shows or the internet.
Unfortunately Kevin Marti didn't choose to include the "extra cooling package" in his break down of options and how many cars were produced with it in his book Mustangs by the Numbers
Haven't found a Marti report or buildsheet of a 68 SB Mustang with that option on it except for 68 GT350 but like 67 GT350's that extra cooling package was part of the standard equipment for those cars as written in the specifications but these were not your regular Mustangs. Still looking through more