Yeah, MN12's and Foxes have different offsets/bolt patterns. I own one of each and tried to swap my Cobra rims to my Mark VIII when i found out the hard way.
But as for center caps. I can visualize a few rims that they might swap....but there are a LOT of stang rims and tbird rims to pick from.
Right off the top of my head, i believe the sunflower 15" T-bird rim and the 17" '93 Cobra share the same center cap. There may be others