Get some Duraglass. That's the easiest stuff to work with. Bondo makes it. Just sand down the area so there is no paint. Then scuff up the area and make your mix and apply. It will get really warm as it hardens. Once it is dry and hardened sand down and apply another layer to fill in any holes or cracks in the first job. Repeat over and over until you get a nice smooth, free of holes or cracks surface. Then repaint. It should hold just fine as long as you don't apply the Duraglass (Bondo glass) to a smooth surface. Always scuff up the area you are about to apply it to. It should come out fine. But if I were you if your gonna fill the GT you should fill the Mustang too. That should make the car look a lot cleaner.

 

ok, thanks.

 

use a poly based plastic filler. it will flex. i was thinking about doing the same thing but covering the whole thing. it's funny how people on this forum think telling you to throw money at it is helping. like you never thought of buying cobra skirts.