Seat upholstery issues

I'm restoring a '64 Valiant and have '65 Dart bucket seat cores. I bought on Ebay what was supposed to be '64 Valiant upholstery and it does not fit the Dart cores. Would the bucket seat cores between the two years / models have been different?

Also, I'm having trouble finding upholstery for the back seat, which I believe to be the original for the car. Anyone know of a vendor that sells back seat upholstery for a '64 Valiant?

As always any advice is much appreciated.