noob needs help with mileage - only 10 MPG!

The absolute best I ever got out of my slant was 15-16 mpg mainly highway in a '74 Duster a904 with 2.76 gears, dropped down to 8-10 when I swapped in an 8.8 with 3.55, pulled it shortly afterward. It was a 77' super six. It had flow issues with the head and never ran properly.