With a Rochester Vacu-jet carb that is stock equipment on a '57 235, it will have very little manifold vacuum at cruise with tall gears. I don't recall the spec's, but I'll bet the power enrichment valve is open on your carb at cruise speed, that causes high fuel consumption. You can experiment with different springs above the piston or snip a bit off a spare spring and see if that makes any difference. Remember, less spring tension delays the valve action, no spring at all defeats it entirely.
Put a vacuum guage on it and drive by it, keeping as high a reading as you can. I'll look in my books for the vacuum values for the carb valve, but I don't recall seeing them for years. Too many books!