I'd take this in one of two ways.
1. They are going to add more modern technology to the 6.4L such as Direct Injection and/or MDS to improve the mileage.
Or
2. They are going to use a modified version of the 4.7L might be used. The 4.7L is supposively going to be updated before long (3 valves per cylinder); perhaps, they are planning on supercharging it, making a larger version, or making a combo of the two.

I personally would doubt this one seriously and I hope this is NOT the route they choose on top of that. The 4.7L is like the Ford Modular design, it doesn't seem that it can get any decent power without forced air induction. The thing that has turned me off of the Mustang is their use of OHC engines. They just seem out of place in a muscle car. I prefer pushrods in muscle cars.

I used to be a big Mustang fan until they dropped the 302/5.0L (actually it was more of a 4.9L but most people don't know that). I loved the '95 Cobra R with the 5.8L/351 Windsor. I wish they would have mass produced that model so then they could have a chance in competiting against the Trans Am and SS. The 4.6L has never gained my respect and neither has the 4.7L.

Hopefully it will be through technology. Also it might be due to the transmission. If I'm not mistaken, I don't think the W5A580 transmission used in the LX cars was designed for this level of torque. I think I read somewhere that the torque limit is somewhere in the mid-400s ft-lbs. I'll discuss this in another topic.