As long as UA connects your bags to AC, you can go to an immigration "bypass" counter which should be only a cursory passport and boarding pass check. YUL isn't that big an airport, so 1:10 is fine. AC actually gives a MCT of only 40 minutes for YUL for a US-Int'l connection. So in ACs view, you should have a solid half hour cushion. On the way back, they want 1:10 but only because you need to preclear US customs.