Shouldn't be much of a problem, especially the 20 mpg provided that you keep the rpms down on the highway. In the recent Car Craft, a guy got 25 mpg out of his 80's Trans Am while pulling off a 12.88 1/4 mile on a mild, N/A car.
However, the new Mustang GT does have potential. If you don't have the money for a GT500, then simply build your own. All you need is a 5.4L engine (preferably one w/ 3V heads to make things cheaper). Once you have that, all your accesories from the 4.6 can be bolted to the 5.4 and then all you need is a blower. The only thing you need custom is the mid-pipe in which any exhaust shop can widen to accomadate the 5.4. Also, a stronger drivetrain will be needed. All in total, it should cost less than the GT500 depending how crazy you go. With 2V heads, stock cam, and ported heads, you can get 290 rwhp and 360 rwlbs*ft of torque.