Hi Fred,
You right on the money to pick up on that.
6' 0” is most common 3’ gauge tie I believe.
The reason I quoted 6'8" was because it was the length of all the new ties that filled all the D&RGW gondolas that were placed in nearly every siding between Chama and Carbon Jct. around 1966 to 1968. Very few were ever used in that territory. I think they were purchased to inflate or fortify the cost of maintaining the narrow gauge for the abandonment hearings. I guess most of the gondolas were shipped to the Silverton line.
I just finished a wet day of surveying a new NG railroad extension and the new ties we bought to lay on it are 6" X 8" X 6'. You are right on.
It would be useful for the modeler reader to have other post here of their measurements of the recent length being use or run over on various NG lines.