Tom,
I got two responces so far. Here they are:
1: Shades of the past, indeed. I went through that with my 67 which was missing a part.
What I did was use a seatback frame from a 72. They are close enough to work. I know, in 67, the headrest was an option which my car didn't have, so it was easier to make it work than possibly with a 68...which MIGHT have had its own headrests...someone else will have to answer that one.
If headrests were an option only in 68, and, thus, not always included, a 70-72 might be able to be reupholstered without headrests and be close enough that no one without a tape measure could tell.
Here's my 67...note that the driver's side back is bigger than the passenger side. I saved the other back to get that fixed when I restore it...the armrest/seatback interface is better with the bigger, newer one anyways.
http://www.landofjohn.com/cutlass/4.jpg
2: I personally consider the seat pattern to be a fairly small difference from 68 to 69. I wanted headrests in my 68, so I had some 71 buckets done in the 69 pattern and did the backseat to match. Yes, the car seat police will bust me, but phoeey to them!
I'll post them as they come:
Dan