My opinion: 1hr is too short.

• you will have to clear Canadian immigration (there's a dedicated connections lane that is usually pretty quick)
• then you have to wait for your bags. I try to avoid checking luggage on AA because it always takes a minimum of 20 minutes (though usually 30-40, priority tags mean nothing) for my bag to appear once I've made it to the reclaim area at YYZ.
• You then have to go upstairs and half-way through the terminal (BA have moved near to the US departures area) and re-check your bags.

It will probably mean that even if you make the flight, your bags won't because it's after the check-in cutoff for accepting bags. There are facilities in Terminal 1 to transfer bags int'l to int'l but not in Terminal 3. All passengers in Terminal 3 have to reclaim their bags here.