I got 12 m.p.g with a '54 239 Y in a 55 F100, T98 trans, a 3.92 rear, and 235/75r15 tires. My brother's '50 F-1 with the 239 flathead get around 16 m.p.g.
The sedans typically get better gas milage than the pickups, probably due to the billboard like aero-dynamics of the trucks. On my trip out west my 53 sedan got between 15 and 22 m.p.g. depending on conditions.