Are the rear seats the same on all 67-69 Darts, or are post cars seats different?
Will a 70-72 rear seat work in a 68?

I'm going to go out on a limb and say post to h/t are the same.
I know 4 dr bottoms are different than 2 dr h/t.
I don't think they changed anything from 67-9 to 72 as far as the frames.
I do know I used a nice 74 4dr top half with a new cover and installed it in a 69 Dart 2 dr h/t with no issues...

In playing with a bunch of 67-72 Valiants this summer I found that the backs are the same but the bottoms are different. The bottoms on all of my post cars are the same as the 4 doors. Took a nice set out a 72 4 door and put them in a 68 post and they fit great. I would assume that Dodge would be the same but it has been a long time since I played with a Dart post. And lets not add Duster seats to the mix.

Thanks for the replies! Its for a 68 Valiant so I know what to start looking for.

Sedan and hardtop are different for the seat tops. Sedan and 4-door are the same, all hardtops are the same, Demon/Duster/Dart sport all interchange but do not on sedans and hardtops.