For your 10/68 Dearborn car I would guess - does make a difference. Different plants and different time periods meant, normally , different ways of attaching
Also Mach I or non-Mach I changed things also
Hoping I guessed that correctly I offer the following (this has been discussed at length on many of the forums over the years)
First - is the radiator support in your car original to the car? If so inspect the trailing edge of the radiator support of pairs of very small holes - staple holes. This may confirm what was originally there.
If the radiator support has been replaced or repaired you are left with going with the "norm"
From what we've been able to figure out - current understanding - during 69 Dearborn production Dearborn used a large stapler to staple the back edge (where the rubber seal is normally double thickness - top and bottom layer) to the radiator support.
http://www.concoursmustang.com/forum/gallery/4/6-220715003737.jpeg[/img]]There is some evidence that sometimes the stapler was out of commission and the workers then relied on some electrical style clips to hold the seal in place.
Related and semi-related prior threads
http://www.concoursmustang.com/forum/index.php?topic=10501.msg62554#msg62554http://www.concoursmustang.com/forum/index.php?topic=1397.msg7082#msg7082Most other treads are related to San Jose and NJ built 69's
IMHO check for evidence and make a choice from there