Rear seat questions

I have a question regarding seat covers and rear seat dimensions for a 69 Dart. I have reason to believe I have 67 Barracuda rear seats. The dimension across the top is around 51" and I have a substantial gap, see pic below. I have a spare spring seat that is 53.5" across the top, which would fill the gap. Can someone confirm the correct size? And can I stretch the seat covers an extra 2+ inches? The stitching lines up perfect on the 51" seat, so I'm guessing they won't.
I may sell the seat shown...