Canadian Route 17 Montreal to Sault Ste Marie

Very viable, it is part of the Trans Canada Highway. There is now a by pass around Montreal that makes life easier. Once you enter Ontario from Quebec you can either stay on 417 to past Ottawa or exit just before Hawkesbury if you want to follow the old highway 17 route that follows the river. If you stay on 417 you will pretty much be on a 4 lane divided highway until a ways past Ottawa where it becomes 2 lanes essentially the rest of the way to S.S.Marie. Just remember your passports.

It sure looks like a viable route to me, albeit at least a few hours longer. It has been a long time since I travelled along the Trans-Canada highway, but I do remember it being a quite nice (scenic) drive and a good enough road.

I-80 would be another alternative, a little longer/slower than I-90 but quicker than going all the way through Canada.