The options to get from YQB to GRB with just one stop are pretty limited -- you'd be looking at United through O'Hare. Looking at this summer the connection in ORD would be just over 2 hours, which would be time to clear customs and get to your connecting flight. Given that your YQB-ORD flight is at 8:00am and that's the first flight of the day I'd think that would be pretty ample time-wise if you chose to go that way. I don't think that's a serious danger, and you've got backup ORD-GRB flights if worse comes to worse. On the way back it's a longer connection in ORD, but since there's just one ORD-YQB flight that's probably the way you want to have a longer connection just to be safe.

If you decide to do YUL you've got a lot more options to GRB, of course. Or you could even fly into ESC or MQT, though with fewer choices.