Complex sentence: In 1978, CPOP successfully nominated the park to the National Register of Historic Places, thus guaranteeing that without approval by the U.S. Department of the Interior, Federal funding could not be used for projects that damaged the park's historic integrity.
Split into simpler sentences: In 1978, Citizens to preserve Overton Park successfully nominated the park to the National Register of Historic Places. Listing on the National Register guaranteed that without approval by the U.S. Department of the Interior, Federal funding could not be used for projects that damaged the park's historic integrity.