Yeah, the 6x heads were used on low compression engines, from '75-'79. But the 6x-8 heads will get a 455+ engine up to 9:1 CR or higher--just right for a pump gas stroker. They're probably the most used iron stroker heads. And the 6x-4 heads, which came on 350's and '77-'79 W72 400's will raise comp even higher. Most sites list the 6x-8 chambers at about 101cc, and the 6x-4 chambers at about 93-94cc. The 6x heads have 2.11/1.66 valves, factory hardened seats, and screw-in rocker studs. They flow well, even unported. Lots of guys just have a GOOD valve job done, using one piece stainless valves, and 7/16 rocker studs. Other guys, with more to spend, pay somebody big bucks, for porting. They'll make over 400hp & around 500ft lbs of torque, unported, on a 455+.

Many engine builders agree that around 9.5:1 CR is the safe max, for iron heads on pump gas. So, the 6x-8 heads would be safer. CR can be adjusted slightly with deck height, head gasket thickness, piston valve reliefs or dish, and shaving some off the heads.