Easiest is getting a GT, and you can't just turbo a vehicle. If you had a 1999-2003 model I would say go for it. It is expensive but man, I've seen a 2002 mustang
punching 600hp Ina. Dyno, twin turbo, however, he had customers intake, custom ported heads and cams, and the hole bang for your bug package. You however have a 3.9 liter as opposed to the 3.8 liter. The best place would be TMA turbo, I'm sure the guy could hook you up. Most of his are compatible with stock, but you will only be able to run it at 7psi (which can give you a significant horsepower boost, at the highest I would say 350) if you do take that rout ABSOLUTELY get a slip differential and new gears, or all you will be doing is burning out.
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