First: Congratulations on using a Ford hardware part number in your post. It seem that many Forum member are afraid to do that.
Now for the good news. The clip nut you need is the one on the right in your picture. It is available from AMK as Ford hardware part number 381863-S32 (note finish, -S32, black or olive drab enamel), AMK stock number B-12248 (6ea for $4).
This is documented in this Forums' library under "Mustang Hardware Spreadsheet 67-68".
Jim
Thanks Jim. I found the item in the spreadsheet that I have been referring to often but I couldn't figure out which AMK part was correct. How do I make the leap from Spreadsheet to AMK? I seem to recall for some items there is often the AMK item listed but not in this case.
Also, I did find AMK part nr B-12251 that lists 45144 in the heading and the clip looks like the one on the left. It's also the Ford part number for the clips on the grille opening finish piece.