Buy a flywheel and clutch assembly and then take the T5 and pedal assembly from the V6 car and put into the GT. Take the V6 cars right front fender, bumper cover, headlight(s) and hood if needed and transfer to the GT. Take it to a body shop and have it repaired and a new top installed. You'll come out way better than swapping the engine to the V6. It's not just engine you'll be swapping; the wiring harness, ECU, radiator, AC lines, exhaust, instrument cluster & Rear Bumper cover (for the "Mustang GT" lettering) would need to be swapped if you go in the other direction. I've been there and done that. The GT will always have a greater resell value and you won't have to put up with the "Oh, it's a swapped V6" comments. Unless the V6 just has that sentimental value that you can't let go...