Thanks everyone for the advice! This is all great info. So, what I'm gathering is that my April start date is way too early for all of the mountain passes and to just bear the heat and start in mid-summer. I'm just worried about being in the northern, mountainous stretches of the Great Divide/Northern Tier/SC in late Sept. Maybe we can start somewhere else (Southern Tier or Pacific Coast) and hit the SC in shape. I knew getting into this that it would be a hard route. The TA, NT, and ST seem easier, but I'm not really to interested in seeing the East coast or the plains and want to experience deep wilderness camping.

Is there a better way of doing this such as North to South? We would really like to be on the road from April to Sept and the SC and Yellowstone area are the only really must do parts for us. The rest we're flexible on. Thanks again!