If you are sitting up front and carrying your bags, you will make it. Canada Customs was very efficient last two times (LHR-YOW). If you checked your bags, you're in trouble as luggage handling in YOW is awful - plain and simple - priority tags or not. The two carrousels in customs clearance are going full tilt as there are usually a couple of transborder flights that arrive close to the LHR 767. US preclearance is quite efficient as well.
Good luck
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I miss Canadian Airlines