67 barracuda fold down rear seat?

You’d be better off using a fold down seat out of a 73+ Duster/Dart Sport. The bracing and floor reinforcement would fit a lot better. The seats are still different and it would require more than little fabrication. But I think the 73+ Duster and Dart Sport fold down seats would be a better start (and likely more plentiful) than the Barracuda stuff.

You can see some of that parts involved here
How To: Fold down seat conversion

That shows the conversion on a Duster that was born with a non-fold down seat. For a Dart there would be some differences with the seat that would have to be addressed.