My one and only (and last I will ever take) flight LGA-YYZ I had call WestJet to get a seat. Remember there are "premium" seats much like EC that they sell for a fee up to 24hrs before the flight. At that time you can choose then for free. So if you do not want to pay you will have to wait til T24 to call.
I also had issues where DL didn't know anything about the WestJet ticket and I could not check in online, and there are no kiosks in LGA, you have to line up to check, don't know what issues you will have as a connection though.