The northern route (Phoenix to Kingman to old Route 66) would be a blast in a GT. I've driven that route a few times in the last few months (in "Taurus Red").
Route 93 between Phoenix and Kingman is smooth and straight, with a few sweepers thrown in for good measure. With a good spotter running ahead, the big 205 would be possible in many places. Old route 66 is a spectacular road to drive on. Up one side of the Black mountains and down the other, with MANY hairpin turns and switchbacks. The entire road is only 1 3/4 lanes wide, so anything over 35 mph is not an option. There are many drop offs and very few guard rails, so it might not be the best for the squemish. But the View from Sitgreaves Pass is spectacular. The only town on this stretch is the old gold mining town of Oatman. The whole town is only about 1/8 mile long. The only draw back here is the wild burros that wander the streets, leaving many "road apples" in their wake.
If you think about taking this route, it might be wise to plan an extra day for the trip, as it would be a shame not to stop and view the many vistas and sites. With luck, I'll be living in Kingman by then, and would love to host a GT pit stop. :thumbsup