I have no experience with the bucket seats.
With the bench seats, it seams there was mounting in the upper metal and in second support maybe 5 - 6 inches below.
Here's a '70 split bench seat back. View attachment 1716257381
67 Bench headrests - which are different than the buckets - at least in shape. View attachment 1716257384
The lower picture of the '67 bench headrests has two brown oval pieces which may go directly under the chrome bezels as guides? If so, maybe that's what my smaller black pieces do also?
I'm at work now, but will have alook when I get home later. Thanks Mattax!