How much elevation is cause for concern? Anyway you travel across the country, you'll be dealing with some degree of elevation.

The lowest possible way is to take I-10 to I-5, but even there, you'll be up over 4500 feet when you cross the continental divide, and there are a few mountain passes over 4,000 feet on I-5, in Southern California and Oregon. Of course, this route also adds nearly 1000 miles of driving to your trip - so you'd also be looking at a minimum of 2 extra days driving, compared to a more direct route.

For comparison sake, Marias pass is at an elevation of 5200 ft. So you're really not up that much higher than taking the lower elevation option.