What time of year did John Lamm do the route?
The original idea of March might be a good compromise as far as the 2 extreme climates one would experience on this route.
Definitely don't want everyone to fry in Death Valley in the summertime. But probably also want to avoid heavy snow in the mountain passes in the winter.
Got to consider the weather conditions beyond just the route itself as well. If your goal is to attract owners from all of the West - WA, OR, UT, CO, ID, etc - many of us could run into closed mountain passes or heavy snow just trying to get to the meet.