"Best overall Lift/Drag ratio" of a plane has nothing to do with "best speed". For a given configuration, there is only one angle of attack for maximum L/D. Suppose for a second that an airplane was at its angle of attack for maximum L/D when also at its maximum straight and level airspeed. How would this airplane have reached maximum level speed? At ANY airspeed less than maximum, the airplane would have more drag than thrust. It would be hard for this airplane to accelerate toward maximum level speed while decelerating.

The point I am trying to make (and others have made it here also) is that best L/D isn't really important for top speed. Airplanes designed for high speed (fighters and racers) don't tend to look like airplanes designed for high L/D (gliders), and not just for structural reasons.