The stud clip you need is 382121-S36, available from AMK as B-12016 but misidentified as 382155-S36 (see below) in their 2014 catalog, plus 45373-S36 nut, also available from AMK as B-12942. You will need to add a sealer available from AMK as B-11248 for a 20 pack, no Ford part number. It's a mastic sealer which may not be correct.
Note: 382155-S is identified as an 8-18X3/4 sheet metal screw in AMK Guide to Ford Fasteners. The 69 Mustang Body Assembly drawing calls out 382121-S36 which agrees with AMK Guide to Ford Fasteners.
Jim