Complex sentence: Many of these stadiums, when used for baseball, had many of the lower-level boxes set back far from the field because they swiveled into position for football and soccer, while the upper-level seats were as far as 600 feet from the plate.
Split into simpler sentences: Many of these stadiums, when used for baseball, had many of the lower-level boxes set back far from the field because they swiveled into position for football and soccer. The upper-level seats were as far as 600 feet from the plate.