After reading your message again I realize that your information wasn't left field at all. Because of the migration path and times and the children's names Isaac is probably related to Levi. It is most interesting that the most likley place for my McMullens is also Grimsby. I will check on the Methodists.
Note! I do not think there were any churches in Saltfleet in 1840. They would have had to go into Hamilton for important events. Today, I suspect they used St. Andrews, tomorrow , who knows!

Yours aye! Mr/Ms Macdonald
I have plenty of kin near Hamilton that do not have Dutch names.