I dunno about 40, it would be cooler temperature-wise than deep south Texas, but the freeway through Oklahoma absolutely sucks. If you really wanna go on 40 I'd recommend getting off of 40 at Amarillo and then angling southeast on Hwy283. I can't remember the actual number of the road, but it takes you from Amarillo to Dallas. Plus, you'd completely miss Oklahoma and their devil-roads.
You would be adding several hours too (don't know the actual number). Either way, it is a fun drive, especially through the desert in a Jeep with nothing but time to kill.