What about this...
http://www.mustangsunlimited.com/SM_...Site/00089.htm

It's for a V6 but I don't know why it wouldn't fit the GT.

 

You could probably make it work, but some of the mounting tabs will have to be cut off or modified

 

I thought the grille portion of the GT and V6 were different sizes?

 

Well, from what I have read. People who install a GT grill in a V6 cover have to trim some of the retaining tabs to make it fit. But, i could be very wrong.

 

The rounded off bottom corners on the 05-09 V6 grilles are radiused differently than the GT and have different mounting tab locations. The 05-09 GT grilles are too squared off on the bottom corners and will not fit properly down inside the radius of the V6 grille opening resulting in a large gap across the bottom. You can place a V6 grille in a GT grille opening but the mounting tabs still will not line up requiring some rigging to secure it in place and there would be a slight but noticable gap in the radius at the bottom corners of the grille opening.

Here is an old thread about putting the Pony Package grille in the GT bumper. You can see the gap difference in these pictures...