Which years seat covers are interchangeable with 66 4 door sedan Dart?

The interchange for covers would be 63-66 as to fit but not pattern. I’m Not aware of any company that makes stock seat covers for an early A 4 door (or wagon). So either good used or custom will be your main options.

I had the same problem for my wagon. I solved it by converting to a stock Barracuda complete interior - rear fold down seat (which bolts in like it was made for it) and a pair of buckets. Upholstery was then no problem - called Legendary for the front and rears and a couple of weeks later they showed up.