I hate to go there, but unless you have a 3.5L Ecoboost laying around, you'd be much better off with a LS1 swap. They cost the same, have 8 cylinders, exhaust that sounds great, and get close to the same fuel economy as a 3.5L.
LS1 = 14-15 mpg tuned
3.5LEB = 17-18 when tuned