Compare the roof line..check if you have a 68 with 69 or latter doors.. or visa versa for some reason.

1968 GM Chevelle/ GTO had door built in vent windows.. 69's did not.

The roof line is different in the rear 1/4 B pillar.

With many cars now coming into or appearing with restorations done in different ways.. its important to get it right if possible. This might help,