Pontiac 6X ID

All this time it was so simple to ID these and I had not one clue, Pete McCarthy educated me in a Pontiac Enthusiast issue. Here is the chart.

500804 large chamber measures 3.630 at point A

500801 small chamber measures 3.212cast #: 76 GM8, 78 GM4, 78 GM7 I have found to be large chamber, cast#: 76 GM3, 77 GM1 to be small

Point B is quick junkyeard test: that little none machined area is almost machined and sometimes is on the small chamber. The large chamber is well indented.