Glad to hear things are progressing.
Mine is a 3127 (just looked under the car). My car is a '79 with a Maier hood and the '86 bubble hatch.
All '79-'86 Capris had the same front bumper covers so you want the 3127. Get the 3127.
The 3129 is for the '83-'86 Mustang only (not Capris) because those bumper covers were just different enough to require a slightly different air dam.
Mine is molded on. I cut the factory bumper cover 1"-2" below the beltline molding. I sanded the bumper cover and inner part of the air dam and applied flexible adhesive. Then I ran big ugly screws through both to hold everything together while the adhesive set. Once the adhesive was fully cured, I removed the screws and filled in the holes left behind. Also ran a bead of the adhesive along the top of the air dam to fill the gap between the air dam and the beltline molding. Then sanded everything until it looked like a single piece.