The red seat is a '67, the black a '68. And, yes, the foam is very different between the two.
You're right of course, it has no markers lights, which means it's a '67! Don't know why I had a mental block on that one, especially since I once owned a '67 Dart too!
That said, '67, '68, '69, it doesn't seem to matter, all I find in my searches on the internet is foam like what I first pictured...
Here's the original seat back foam. As you say, QUITE the difference!
And here you can see how the bolster is shaped--thicker than the seat back itself, and NOTHING Like the flat foam I have to work with!
Guess I better invest in a carving knife! After I check what was recommended above of course. But I'm not holding out a lot of hope...
Thanks everybody for your help, and I apologize for posting this in the wrong spot to start off!