Should be pretty straightforward except for the rear bumper which needs to be cut where the left tailpipe sticks out courtesy of the V6's single exhaust setup.
Are you doing this because of side exhaust? If so, you might want to consider that the V6 bumper stops noticeably higher than the GT bumper. It really looks funny with the stock GT side skirts, but if you have aftermarket skirts, you really don't want to do it.
Not really side exhaust yet. Kooks headers running through Magnaflow cats then dumping out. I was going to just paint my side skirts like the v6's or see if I could just trade those too. I'm just trying to look like a nice v6. No bling or crazy accessories just mustang.
Anybody with a torch red v6 that wants to look like a v8 lemme know I'll swap/sell it all!
I'm fairly certain you'll need the foam. It's not extra foam, but differenty shaped foam. Call your dealer and ask. It will take a few minuntes to find out for sure. The GT and V6 cars have a different shape to the bumpers which are backed by the foam.