Simply adding a turbo will not increase your MPG. The reason the Ford Ecoboost does so well is it's a smaller engine. That's where turbos do well. They allow a smaller engine to be more fuel efficient at low power demands, and then when it's needed, they can make the same HP as a larger NA engine.

From what you've said I think you would be better off with an NA 4.8 or 5.3 and dropping the gear down a bit. A 5.3 with small Crane cam will make 100+ more HP than your current 350 will and more torque as well, PLUS gets you up around 20mpg.