I suspect that the OP might have one of the special sale fares from Canada to China, probably in business class. That could also affect treatment in IROPs as Z fare class inventory is unlikely to be available.

ADDED. I suspect, but don't know, that missing the flight due to a delay on DL would be different from a delay on another carrier as the latter case could lead to a lot of finger pointing about whether Westjet or DL is responsible for trying to help.

ADDED AGAIN. Remember that you go immigration and customs twice at YYZ. The transborder outgoing lines for USA BCP immigration and customs processing Plus security can be very long. Also, you must check in and drop bags at least an hour before scheduled departure time.

Does the flight you're considering leave LGA at 6:30 am or arrive in Canada at that time?