1h25 is tight. You'll have to leave the international area and get into the "Connections" line at immigration (which is just allows you to get to the front of the normal line). You'll then connect to the flight to JFK. Right now, the DL flights are from the RJ gates and is at the very far end of the "USA" terminal and takes about a 10 minute walk from security. On August 19, the new "USA" area opens up, I don't know how that changes the whole process but hopefully it will make it a bit easier.

Since missing the connection would most likely be immigration/customs related, DL isn't required to put you on another carrier. You'd probably be stuck in Montreal for the night. With that said, the afternoon flight to JFK tends to get delayed. I flew it last week and was delayed from a 4:20 to 6:00 PM but eventually got out at around 5:30 PM.