DNF, Where wind is not a factor, there is no reason to fly below MRC (Max range cruise) which equals minimum fuel used. LRC (Long range cruise) is defined to be the speed at which you would get 99% of the range of MRC. I'm guessing, but typically perhaps .02 mach faster that MRC (I'm not an airline pilot). The most economical speed will be where the sum of the cost of time and the cost of fuel are a minimum. This is where the cost index calculation comes into play. I can't really elaborate as I haven't read up on it. There is an airbus document available on the net somewhere called "getting to grips with cost index" which if you do a google search I'm sure you'd find.
However, if you're flying behind the drag curve, as you say, then not only are you below MRC speed, but your also at a speed below minimum drag speed. While this speed is readily available to modern airliners via a "green dot" annunciation, I'm not sure the 767 would have one. And if you were to decelerate much below minimum drag speed, it is possible you may be unable to maintain this speed, especially at high altitudes where thrust is limited.
Hawk