[Tony_Bacala]

Doesn't look like it. This piece comes with a separate neck piece as well, to show his skin on the neck. On the other armors, that neck piece doesn't pop out (easily, on purpose).

I checked the ball joints, and the MK 6 has 2 large balls. On the Stark head, it's 2 small balls. So 1 to 1, head to neck - it isn't working.

If you could some how remove the neck piece on the other armors, that MAY** be on the same ball joint size as this neck. Which would allow for a full replacement. Colors may be off with the armor tho like that. It's requiring too much pressure to try that here, so I'm stopping, you'll have to experiment from there.