* The Monte Carlo was introduced in 1970. It was a completely new model for the year.
* The LS6 engine was exclusive to the Chevrolet Chevelle and Corvette in 1970. It was a high-performance, 454 cubic-inch V8, only available in those models.
While a Monte Carlo could be modified to have an LS6 engine later on, it wasn't an option when the car was first released.