"...Are the 6x heads that much better than 4x??"
I think the biggest advantage of the 6X heads is that they are less prone to cracking, between the valve seats. There have been LOTS of reports of cracked 4X heads. Some say it was because of the seat hardening process used on the 4X heads, which was improved on the 6X heads.
Don't know the real reason why, but I've read about the 4X cracking problem on several Pontiac forums.
http://forums.maxperformanceinc.com/...66#post5928466
https://www.hotrod.com/articles/0312...-racing-heads/