actually it's the motor that dictate the mounts you need...and those clamshells don't have discernible differences. We built a camaro super stock for this year and the front clip from the 1970 camaro wasn't in great shape so we used a front clip from a firebird. The pontiac engines were a mid size block (not a big block like some think) and although those clamshells looked like the ones for a SBC and the eurothane fit the clamshells the firebird clamshells wouldn't work...when you put the checy and pontiac clamshells beside each other you can see the subtle differences but they are real close,,,