How can you improve fuel mileage on 454 1984 GMC dually 373 gear 3 speed auto?

Well - I have a 1984 gmc dually that is a big truck, and im only getting about 10 mpg. The way u can increase your gas milage is to get flow masters and then get your truck fuel injected, you can also try to gettting a bit bigger tires, in know what your thinking getting bigger tires will decrese your gas milage but really infact it will give about 2 more mpg,