I just got halo headrests that are supposed to fit all 79 to 83 Mustang LX and GT's, removed the headrest from the lx sport seat and found that the hole and entire plastic sliding mechanism is smaller and has a slot running down the center. I have seen many of these seats with halo headrests and was unaware of any differences between years or need for modification. Before I go machining down the aluminum on the headrest or shaving and cutting the mechanism, am I missing something here?