I don't know what ACA has planned for the route between Ash Fork - AZ and Williams - AZ. As far as I know the original Route 66 is completely replaced by Interstate 40. To bypass this 19 mile long section involving 1600 ft of climbing, I took today County Rd 142, also signed as Double A Ranch Rd, out of Ash Fork. After 4.8 mi the road changes to FR-124 but keeps its name Double A Ranch Rd. It soon becomes a dirt road with a solid red-brick surface, until it gets paved again a couple of miles before Williams. It is a rolling, very scenic road. Of course it also involves at least 1600 ft of climbing and totals 29 miles, but in a quiet setting without diesel fumes. Dirt roads are also less risky for punctures than the shoulders of Interstates. Part of FR-124 runs along the Santa Fe railway. I saw 8 freight trains passing in the 2 hours I was on this road, more than the number of cars and motorcycles. There is no way of getting lost if you keep the two numbers County Rd 142 and FR-124 in mind, because the main road is much broader and much better maintained than the side roads ending on it. While a dirt road may not be liked by all cyclists, I would recommend it at least for eastbound Route 66 travel. Westbounders might try to outpace the heavy trucks on the Interstate.
I've cycled Route 66 from Flagstaff, AZ to Barstow, CA on this tour: http://www.bikingbrian.com/tag/route66tour2010/
I've done the Williams to Ash Fork section twice. The first time was late in the day, and we had such a strong headwind that we couldn't go faster than 15 mph - even on a fully loaded tandem. I wish we were going uphill eastbound to have a tailwind.
The second time was late morning, and I had no such problems with the wind. In general, I've found the I-40 shoulders in Arizona to be quite excellent.
But I-40 shoulders in New Mexico, well that's another story. I was on a business trip to Gallup, NM and saw the abysmal condition of the shoulder when driving back to Albuquerque. But then I saw this route <http://www.bicyclemaps.org/rt66/Route_66/Welcome.html> which looked encouraging. Following an older Route 66 alignment is "legitimate" to me because it is still in the spirit of seeing the history.
I put Route 66 east of Flagstaff on hold because I just wasn't excited about riding large sections of I-40, even with the possibility of combining with a business trip to NM. But I'm encouraged that there may be some creative alternates, so I am waiting for the maps with excitement!