User blog comment:IamJakuhoRaikoben/Chapter 371: Tartarus Arc: Part 2 - Song of the Heavenly Dragon, Review./@comment-121.54.32.145-20140216105353/@comment-2214433-20140218120435

It's not a speculation. Chaos posted on one of the blogs that Mashima confirmed back then that these 'part' titles cover the entire batch of chapters.