For riding within Quebec, refer to the Route Verte page. http://www.routeverte.com/routeverte_carte/index.php?langue=en
The direct route from Montreal to NYC is via Route Verte 1 & 2 to the US border, then using New York State Bicycle Route 9. I have ridden from Albany to NYC and this is a good route. Most roads have wide shoulders and the scenery is good.
New York bike routes have an interactive map at https://www.dot.ny.gov/bicycle/maps
, click on "Use The Maps" tab.
As an alternative, you could ride through the Adirondack Mountains. The high peaks region around Keene has some short but challenging climbs, but most of the area has more gradual climbs. The scenery is great. Route 30 from Malone to Northville is a great ride.
Another choice would be to ride through Vermont to Fair Haven, VT and Whitehall, NY. It is flatter than the New York side and still very pretty, with nice views of Lake Placid and the mountains in NY and VT. Switch to NY Bike 9 at Whitehall.