[KK]

670 heads have a closed chamber design and behave differently than open chamber design heads. They generally require more timing than open chamber. I wouldn't be afraid to run them on a 400 with pump gas and performance cam. 455 would require some blend of fuel or race gas. Polishing the combustion chambers goes a long way to prevent detonation. 670's have a really good short turn radius and combine that with a little porting and you can make decent power.