Since you have a 4.6 forget about turbos it just is not worth it on these motors. The 4v motors are the way to go from the start IMO but since you already have the GT you would be wasting money and time to pull it out and put in a 4v since you could put a supercharger on it for much less and have more power. Just dont plan on running more than 8lbs of boost on the stock rods.