I'm from Winnipeg, so it's extremely difficult for me to think the border any farther north, especially having been through some of the territory north of the line on this map.
Western Canada being part of Canada is already very illogical anyways, and forcing the border past the south end of Lake Winnipeg would cause too much economic strain to make a united East and West feasible, especially with cutting out a vast majority of the arable land of Western Canada too.
The problem is that it wouldn't be one country. It came to be in it's own sense because of westward expansion, so if it were to become its own country in that sense, it would just be Western Canada. Whereas if it was formed from the West, formed from the West and split up, or formed organically like most nations, the name(s) would be vastly different.
So yeah, it really depends on how much change in history you really want.