ALWAYS mag heads and blocks before you sink any money into them.
The term "better" heads is completely a matter of opinion. Pete McCarthy rated the 670 as one of the best flowing heads at low- to mid-lift.
My opinion: Personally, I really like them because you can open the chambers to match the style of the later ram-air heads and get them up to about 82-85 cc, which tends to be perfect on a 400. Add ram air length valves and port them to your heart's content, and you'd be hard pressed to find a better street head for a 400. The down side to 670's is that stupid A.I.R. boss built into the heads.
