5500 Tops and not for very long.
I have a 69 Weimann custom with a 455 Olds, eldebrock system matched carb, intake and cam, place diverter, and blueprinted pump my Marine performace designs and the best I have EVER run was 63. And that was on Mohave (river) below Hoover dam on a cool day in cold water going WITH the current. Most of the time is 55-60 MPH. So yeah 67 is a little hard to believe.