i do not think it matters much, my god fathers 1969 cobra came with none when he bought it new and the dealer just took a guess and put them on. i was told years ago many of the emblems were stolen off of them when they sat in the train yards in 1969.
I'd just find a nice old factory ad photo and put them as close to that as you can. Besides, things like that were way looser back then. Remember, these cars were built by people, not robots like they are today.
Sounds like a cool project, is it an original Cobra car or a "tribute car"?