4th gen. I did question whether X-body (such as 75-9 Skylark) mounts and accessory drive would swap to an L67 because I don't know. I am a Pontiac V8 man more than a 3800 freak.
The OP is ignoring me so I'm out. Put aluminum heads on a 455 with a nice cam, paint the whole thing blue, done. Leave 326 emblems on it and lie, the engines look the same from up top. Building a 500 HP 326 would cost more and make less sense than any of the options mentioned so far. Pass the LS bong to the next guy.