Complex sentence: It was introduced in 1964 model year as an option package for U.S. sold F-85 and Cutlass models, became a model in its own right from 1968 to 1971, then reverted to an option through the mid- 1970s.
Split into simpler sentences: It was introduced as an option package for F-85 and Cutlass models sold in the United States beginning with the 1964 model year. It became a model in its own right from 1968 to 1971, then reverted to an option through the mid- 1970s.