Complex sentence: After differing with the administration there on certain key issues, he left this post in 1974 and began his own Yeshiva as the "Rosh Yeshiva" of Yeshivas Brisk in Chicago, which claimed to be an American incarnation of the Brisk yeshivas and methods.
Split into simpler sentences: After differing with the administration there on certain key issues, he left this post in 1974 and began his own Yeshiva. He then became the "Rosh Yeshiva" of Yeshivas Brisk in Chicago which claimed to be an American incarnation of the Brisk yeshivas and methods.