Some Suggestions

I see where the difficulty is. I agree that the overview is where you should put it. Perhaps state "Late modern North American archives of Canada and the United States..." or you could have a section for Mexico and simply have a link to unfilled new page, though that's highly misleading at this point.

Having said that, to stick with Wiki format, you may want to remove the title from the "Overview" section as most Wikis tend to jump into a definition of what the page is about without it being a separate section.