As I have said many times before, a route that attempts to follow Historic Route 66 religiously will be very disappointing for cyclists. The two main reasons are 1) There are many places where I-40 or other interstates have been built over the old route with no parallel route - so you have to ride on the interstate. 2) Where there is a parallel road, it is often a service road to the interstate with all the noise and ugliness of the interstate - plus often a pretty lousy riding surface.
I have argued for a route that attempts to recreate the "feel" of Historic Route 66 in New Mexico and Arizona rather than the actual physical route. (Not to mention that the physical route changed many times.) This would involve, perhaps, riding from Abiquiu to Cuba to Crownpoint to Gallup, then riding via Window Rock and Second Mesa to Tuba City and the Grand Canyon.