From the comments, it seems my mileage is (or) was average. A stock 350 (crate motor) and stock exhaust manifold 81K10 "highway" driving from Central AL to West LA Driving about 75 MPH I got 12 MPG. About a 7 hour drive without much stopping (only gas).
My MPG may change when I complete my new engine build of a 357, 2.02 intake heads, a larger cam, and long tube headers.