On I-40 you will have a pretty good climb just west of Williams and the construction caused the road to go down to one lane for a few miles. We did it yesterday and had no trouble but just wanted you to be aware. Flagstaff is at about 7,000 feet and Kingman is at a little over 3,000 feet!
If you go on down to I-10 on US 95, it will lower the elevation changes and also increase the heat as well as sending you through Phoenix and El Paso. When you are going across country you just have to pick your poison! :B