Actually DL Widget Head, YVR and MIA might be more like at least 5.5-6 hours apart. Going eastbound, yes, an A319 might be able to do that. But it may require a fuel stop in the other direction because of headwinds. Apparently, this seems to happen on the supposedly nonstop YVR-YHZ route and even the YYC-YHZ route, but I'm not too sure.
If the A319's going to be used on that route, I don't know if AC would place stricter baggage and cargo restrictions in order to optimize the range, but then again, I wouldn't be too surprised. Still, it's not a bad way for AC to go as far as transborder routes are concerned.