I've read that putting big HP into a Mustang requires big changes not only in suspension, brakes and cooling, but also the basic structure of the car.
Anyways, I'd just go with the FRP Aluminator N/A engine, which is good for at least 500HP, and put real money into suspension, brakes, differential, cooling, chassis stiffening, and perhaps a different transmission. It's not just how much power you make, but how you get it to the road and use it.
Which, I'm guessing you already know.
2012 GT Premium, Brembo PP, MT82, 3.73s, Blk/Blk. Bama 93R, Koni Yellow Rear Shocks, Hawk Ceramic pads, Luxurious & Plush Floor Mats