Will 6X Pontiac heads that came out on 400 and 455 engines fit a 350 engine?

No, 6X heads from a Pontiac 400 or 455 will not directly fit a Pontiac 350. While they share the same bolt pattern, the deck height difference between the 350 and the larger engines is significant enough to create interference. You would need significant modification, likely including extensive machining of the heads and/or block, to even attempt to make them work, and it wouldn't be a straightforward or easily accomplished task. It's generally not considered a practical swap. Stick with heads designed for the 350.