If you have countershaft sprockets that will both work on that length of chain, you can go small in the mountains and put the big one on in the open country. A countershaft sprocket is a lot easier to change (and carry!) than the big ring!
I agree, you want slow speed on the mountains...there are some technical rocky patches and places you NEED to go slow!
There are several RR's with DR400's out there... (probably the most popular flavor of TAT bike right now!)