From research on this forum, I've learned the difference between 906 and 452 heads is hardened valve seats and I believe there was 2 cc's less volume on the 906 heads. I believe they flowed about the same. Can someone confirm? So my question is, how do both stack up against the 346 heads on my '73 440 relative to flow? I think the 346 heads have the same volume as the 452 heads (81.5 cc's) but the 906 heads are smaller at 79.5 cc's?