Actually I believe it'll be a complete swap if you change the grille and the front bumper, the only reason why I know is because two weeks after getting my 09' V6 I got backed into and had to replace my entire front bumper, I was going to switch to a GT, and that's when they told me that I would have to swap the grille too, I didn't do it considering I wasn't the one paying for it, from the dealership the front bumper for a GT brand new and painted will cost between 7 and 8 hundred I can't remember the exact cost though. This is of course just the bumper alone so I would look at some swap yards.
You can use the GT grille but it is not a direct fit. I put an 05 GT grille in my FIL's 05 V-6. I ended up cutting all of the tabs off the back of the grille and putting some 3-M double sided tape on the bottom of it. The sides of the bumper that go around the grill are wider on a GT. You have to squeeze the GT grille into the 6cyl grille opening in the bumper. Once in place, I drilled two small holes through the upper sides of the grille and through the bumper cover and secured it with pop-rivets. I painted them black and you can't see them. That has held the grille in place for a year now. I did not have to adjust the hood latch or hood bumpers at all. It does take a little more force to close the hood then before. The GT grille looks really good in the 6cyl bumper! He has had several compliments! I ordered the grille for his car from NewTakeOff.com. To make the foglights work, I used the harness from street scene. Great harness, and easy to use, but it will not allow you to run foglights and parking lights. I am sure that I can re-wire just haven't had the time. Hope that helps a little!