help w/ shelby decklid on a 67 fastback

It has nothing to do with the quality of the 'glass. I've purchased 'glass from Branda, Maier and Mustangs Unltd and the Mustangs Unltd stuff was by far the nicest. The problem is the lack of the Shelby-style taillight panel, which is flatter than the concave Mustang panel. I've seen several people install the Shelby decklid and end caps and just leave the gap, but I wouldn't.

I used the Shelby taillight panel, so our situations aren't the same. But in my opinion, you're on the right track. If you've never worked with fiberglass before, don't worry, it's easy. Your local paint store will likely have the mat and resin as well as enough information to get you started. Just make sure to grind off the gel-coat before you add 'glass, and use a good epoxy primer to seal the raw fiberglass when you're done (I used House of Kolor with great results) to keep from seeing the 'glass mat pattern in your finished paint and you'll be fine. As far as difference between '67 and '68, the only difference in the Shelby decklids, is that the rear edge of a '67 is approximately 1/4" longer than the '68. You have to look very closely to see it, but one way to tell is that you can see the rivets or screws holding the '68 taillight panel onto the steel Mustang panel when they're closed.