I am from montreal and have done the trip to and from the US at least a dozen times.
Like some have said, do not fly into CYUL, landing fees in the hundreds of dollars. I would clear customs either in Cornwall CYCC, Bromont CZBM or Mirabel CYMX. Call the canpass number and let them give you some options but those are the ones i use. If CYHU is an option for customs, go there and leave your plane there at the Pascan FBO.
Someone mentioned something about not saying Mooney Nxxxx, I say Mooney Nxxx all the time and they are not phased by it. In fact, they will ask you what type you are flying and that just saves some time. Some people speak french on the radio in quebec which i find to be completely ridiculous given that lots of pilots on the radio can't understand it. Other than that, flying in Canada is virtually identical to flying in the states. If flying into an uncontrolled field, the pattern entry procedure is different.
In terms of xpndr, only mode c is required, nothing else.
When you land, you call the same canpass number (you don't need to be a canpass member, but that phone number is for all ga arrivals) and tell them you've landed, that's literally it.
Going back to the US is the most annoying part. Filing eapis, calling customs at least 2 hours before, making an appointment, advising if the time changes, etc...
Edit: just realized you said you are flying for work, if work is paying your landing fee then by all means, go to CYUL and spend $$$ parking at one of the FBOs there.