Very interesting, always wondered about that. The critical cylinders in an inline 6 are cyls 1&6 since there is only 240deg. crank rotation from the previous cylinders (2&5).
Since I am planning to run the inline6 with a GM ECU (same as 4.3L) I would need to place the O2s in the cyl.1 and cyl.4 exhaust stream.
