The issue is that a 767 is only -so- wide, therefore 6 times x (seats) plus 2 times y (aisle) can't be more than the total width.

5 across in a 767 (AC, CP) is always better than 6 across (all other N. A. business classes)

6 across in a DC10 (i.e. CO) is better than 6 across in a 767