I designed this set for MCG and I'm sorry you have had some trouble with it. It appears you have butted the Photoetch piece to the edge of the dash instead of attaching it UNDER the dash. The speaker grille needs to be much closer to the dash pad for it to fit as the unmodified kit piece.
From the instructions:
"Included in this package are several pieces to enhance the dash supplied in the AMT kit. The top of the dash of the real car is metal with a crash pad on top of it. The area on the top of the dash that is not the crash pad needs to be replaced with part no.26. The dash has a very faint line of demarcation that is a good guide of where to cut to add the metal top. Part no. 26 is designed to be glued UNDER the dash so that a lip is left as the edge of the crash pad."
I've included the illustration that shows where to approximately cut the dashboard and a photo of the real car that sort of shows how this works (hard to find a shot of the dash through the windshield).
Also, this piece works well in the 1st gen Camaros as well. I used it in my build of this '69 Camaro and it greatly improved the detail seen through the windshield.