Shelby gt500 is high 40,000s 48,*** I think. Nice cars, defenatly falling behind teh vette and the viper though. Those three are all similarly priced vehicles, and the vette is the fastest, but the viper has the best acceleration, and the GT500, well its a nice car, it just cant keep up with the them. Not even teh GT500 King of the Road version. 500HP its nice, but if you are really going to spend that much on a car, save a little more, and get a viper, so much sweeter, much much rarer, and 600HP. Pllus a v10. The mustang is not really a gas guzzler tho, you can still get about 14 MPG city. Go for teh viper, or teh vette, both nice cars.
If you want a car that is truly the best american supercar, go for the Ford GT40, it is one sweet car. V8, rearmounted engine, beats some ferraris in races. Ford built it when they were buying ferrari, and Ferrari backed out of the deal at the last minute, so ford designed the GT, and it beat all ferraris for years to come. Then, in the future, they made the GT70, and the GT90. Both were failures, so they finally started making the GT40 again. And it was a beast, still is, beat ferraris again, like Fords goal, for only a fraction of the price of a ferrari. It is probably out of your pice range, but it is a sweet car.
Just a side note, the big block, is in chevy cars, not Ford.