The problem I had with mine was that the rear seat panel bulged out because of the vertical supports that were mounted to the frame. Seemed like the supports were not flush enough when mounted and stuck out some. Looked bad.
Mine is a ‘68 and I did not mount the headrests, so maybe it wasn’t done well by the first upholsterer. Now that they are out, all of that looks much better. Something to look out for. Again not sure if seats with the headrests had a different frame. I know some one with a 3.5, and his seats are different. The back panel does not sit quite as deep as mine.
The other reason I took them out is that I really disliked how “large” the headrests looked inside the coupe. Without them, there is a nice open area above the seats and all around.