Complex sentence: They were the first cars to be built in the plant for export (with over half of the C301's components made there) and the largest order manufactured in the United States for over twenty years, as part of the Buy American Act requirements.
Split into simpler sentences: They were the first cars to be built in the Plant for export, and the largest order manufactured in the United States for over twenty years. Over half of the C301's components were made in America, as part of the Buy American Act requirements.