Lane.....Been doing some looking today and found a 03D car with a couple of photos that show something.
This photo shows that the #6 screw and #8 nut, shown on the AIM, were only used on the two end holes. If you look at the left side of the photo you can see the left side of the grille setting on top of a U-Nut (#8) that is attached to the valence panel....this is where the screw (#6) is inserted. Then you can look toward the center of the photo and see the heads of two screws that are attaching the grille to the valence.....but you can't see the nuts underneath. Apparently these are the screws and nuts you are in question about and they don't show up anywhere on the AIM, that I can find anyway.
This photo is a closer view of the two screws....looks to me that they are phillips head screws of some type.
This is all I have found today...maybe some one else will see this and take a photo of the nuts underneath. But this could also be a collision repair.....don't know for sure.
BTW.....we need to give Lucas Restorations credit for these photos. If you want to see the whole gallery of the restoration of this car then go to this link. Look for Jay's 68 SS-RS (1), (2), (3), & (4) Well, if you want, you can look at all of them. https://picasaweb.google.com/LucasRestorations