Ford and Mercury cars with 390 s regularly got 17 MPG on a freeway drive with 3.0 gears and a stock engine...old people driving
With todays modern cams and with the new fuel injection systems you should be able to make 18-20 mpg with your truck ..IF you dont raise it up in the Air ..keep it stock height a good mileage cam and don't go too wide a tire.
and Biggest thing of all KEEP your right foot off the loud Pedal
Make sure you have a good free flowing dual exhaustand a high gear in final drive
And ofcourse I am assuming a 2wd 1/2 ton ??
If your truck is a 3/4 ton or larger....forget it 15 mpg would be good