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