I see referenced in several places that later 6x heads had redesigned intake and exhaust ports. I see there is an external difference in the external appearance of the siamesed center exhaust ports from photographs but dont see an external or visible internal difference between my 62 or 670 exhaust or intake port passages. Anyone know the actual difference if any? May run up to ME and pickup a pair for experimentation.I am setting up a 430CI on 3" mains with a modified factory crank and am trying to gather porting information. Have Pete McCarthy's info, Jim Hands book and HO Racing's original porting instructions. I see Pete says some later heads 96,7k3 with larger (than 72) chambers deepened them without changing short turn radius making the flow at upper rpms deficient. I see that Jim says the 6x was a redesign that flows as well as any Pontiac d-port so maybe that short turn was returned and the chamber reshaped or something. Will be grinding inside my sandblast chamber on a set of something I can get milled to acheive my target SCR and DCR. Have sets of 5C,7J2,11 and 62 on hand to acheive practically any I desire. 6x seems to be popular for some reason was wondering what exactly.