In my younger days, dads 57 3100 that we put a 327 in with a carter WCFB carburetor, 4sp and stock rear gears, got 24 mpg on the road. If everything is done right, I don't see why a 235 old truck can't get into the 30 mpg area or better. And its not going to be all stock as far as internals and gearing.
Though a small block sure gives more choices of efficiency improvements over the 235.
