MONTREAL—Generations of toil at dizzying heights have culminated in this towering achievement.

Ironworkers from a Mohawk community were part of the team that installed the final section of spire at the top of the new One World Trade Center in New York last month.

John McGowan was one of those involved as he and colleagues wrote a special page in a history filled with high-level triumph, and also with tragedy.

“It was a clear nice day. It couldn’t have been a nicer day,” said the 48-year-old resident of Kahnawake, Que., near Montreal....