I’m not sure you’d would have the room for that width, regardless of the length of the boulevard. I can think of a number of choke points from the Humber to the Don. And if you are going to have a boulevard you will need to provision for walking, cycling, ground level rapid transit of some form.
Even if you’re buried it (think Ville-Marie in Mtl.), I believe you would still face those same issues, although ‘express‘ traffic could be routed below a more local boulevard style roadway.
I guess the side benefit would be that the route as it exists now is a fully committed series of roadways, a pretty sterile Enviroment, and you would not have the issues of ripping apart communities, as Montreal went through as Ville-Marie was built.