If you buy a +32 or +35 bag, you should be fine. You could even buy a +40 or +45 bag assuming you have a good insulated sleeping mat and are willing to wear extra clothes on the coldest nights. Also, you usually have options of where you sleep, so if you plan to sleep in lower elevations as much as possible, you can avoid a lot of cold. Although it might be cold on top of Hoosier or McKenzie Pass, you're not going to be sleeping there. The coldest weather you'll see will probably be in Colorado (Guffy, Fairplay, Frisco) or in Yellowstone.
Of course it depends on the year, and who knows what the weather next year will be. Typically, people have more problems with heat on the TA than cold, but you're starting a bit earlier and traveling a bit faster than most, so you have a good chance of avoiding a lot of the heat.
It also depends on how cold you sleep. The temperature rating of a sleeping bag is only a rough guide. If experience tells you that you get colder on camping trips than your companions, you'll want a warmer bag.