Id also like to know that too. I have 1969 seats that I want to put into my 68, and I need the rear seat upholstery for my 1969 seats and my 68 fastback seat. Is the rear seats the same?
I think Mustang and Fords just did a tech on this or it may have been Mustang Monthly. I know they said an earlier seat will fit a 69-70 with a few changes.
The article from June issue of Mustang Monthly states theoretically the fold-downs from 65-70 are all the same, there are few non functional differences stated in the article. The trap door is also said to be interchangeable.