This note may be a little late but have you considered an engine from a 2011-2014 F-150 that uses the same 5L Coyote engine (bottom end is exactly the same) as the Mustang GT of the same years with only minor differences in the HP and high end torque? I just got myself one out of a rear ended F-150 and it is exactly the same with only 30HP difference to the wheels but has more low end torque than the GT 5L coyote engine. The difference is a little different compression ratio but for the most part. I expect to use mine as a daily driver so even if I did nothing I would have 400 RWHP due to the lighter weight of the car even if the motor specs at 360 in the truck.
Here is a link on the motor in the F-150, you can also see what the GT uses as well.
https://en.wikipedia.org/wiki/Ford_Modular_engine look about half way down the page for the 5L 4V motor. You get 412 hp (307 kW) at 6500 rpm, 390 lb·ft (529 N·m) at 4250 rpm from the 2011-12 GT, and you get 360 hp (268 kW) at 5500 rpm, 380 lb·ft (515 N·m) at 4250 rpm from the F-150. from a spec standpoint I have found virtually no difference in putting GT or even GT Cobra cams, lifters etc... on this motors foundation. Here is a picture of the motor once we got in on the engine stand to start working on it.
20150910_205237.jpg The engine cost me only $1700 USD, all I really need to do is add a transmission and a few other parts and I could start it and put it in for under about 5K!
We expect that once complete we will have about 480-520 RWHP before we even think about putting a supercharger on it, if not more!
if interested I have more links with more info if you would like more info.