Well I got a V6 grille to work with my GT bumper for about a year and a half.
The big problems are:
The 6cyl Grill is smaller. If you run chrome trim around the outside edge (the kind with the double stick tape on the back it fills the gap nice.
The center lower tab on the v6 Grille. You will need to cut a slit in the GT bumper (It is a tab that holds the grille in and wont be seen from the outside) with a dremel.
Zip ties will hold it all in place. You will need to strap the grille from the sides around the lower portions of your headlight bucket braces. The only thing that will hide the zip ties are headlight extension covers.
I had mine this way for well over a year and it worked fine and did not move at all. This was also doing well over 100MPH on quite a few occasions.
I eventually got a great great deal on the Agent 47 grille and put that one in so I did remove the V6 Grille.
My bumper cover is still modified with the slit and you might be able to see it with the slit I cut into the bumper cover if you get underneath the car..... If you want to check it out next time LMK.