I'm pretty sure Federal Regs for mandated headrests were not mandatory until the 1970 car model year. that is why for mopar, the "high-back" bucket seats were in everything. the higher back fulfilled the "headrest" safety regulation.
That actually makes Perfect Sense, knowing how Ma Mopar liked to save a Lil cash on the Manufacturer's side, and make ALOT of stuff interchangeable, DEFINITELY AIN'T knockin' Her, cause that Bread went to R@D, and Engineering!