I know this is an older thread but I'm looking at a similar situation next Friday. A six hour layover in Montreal between 2pm and 8pm. I was thinking that a rental car may be the best way to get quickly and relatively cheaply from the airport to Old Montreal. There are two of us so its only a few dollars more than the bus but a lot cheaper than a cab. Does that sound do-able. Grab a rental car, head to Old Montreal, wander around and have a meal and head back to the airport.

Thanks for you help
Mitch