The 454 LS6s are the rarest Chevelles out there. They were only made in 1970 and Chevy only produced 20 of them. Chevelle SS came in one of two options at this time. A person could get a RPO225 which came with a 402 cubic inch engine or a RPO215 which came with a 440 cubic inch engine. The LS6 was an upgrade that was only available on RPO215s. The LS6 upgrade added 100 horsepower to the RPO215, allowing it to produce a total of 560 HP.