I've gone northbound on this route, and I would recommend going south-north in July. Not for winds, but for daylight, as you'll be "chasing daylight" the whole way north. For example, if you started in Missoula, sunset on July 1st is 9:34PM. Sunset in Edmonton for July 31st is 9:31PM. At the rate you'll be going northbound, you'll pretty much have that consistent sunset time, and will have nice long days for riding.