6X heads will have a "4" or an "8" stamped into the machined pad indicated
by the circle. The 6X heads stamped "4" are usually the 89~95cc heads.
The heads stamped with the "8" are usually the 98~100cc heads. I'm
hesitant to state an exact cc for these heads because people have reported
varying chamber sizes. I believe Pontiac used the 6X castings for a multitude
of applications and machined them for the application. The ONLY way
to be sure what you have is to measure the chamber size yourself.
But if you find a set of 6X-4's you can be pretty confident you have the
smaller chambered set. The 6X-4's are typically found on 350's and
W72 400's.