I've seen a few different cammed 04 GTO's without heads or intake put down slightly over 400. Granted they were both 6 speeds on a dynojet but is very achievable without heads.

On another note i was completely unaware that a fast 102 wasn't much of a hp gain over the ls6 on a 346. I have always been under the impression that it can be anywhere from 15-20 rwhp gains. Especially with a cam and afr 210s.