A friend needs a pair of bucket seats for a 71 Demon, we have found a pair of F body seats. How much different are the seat frames, would like to use orig style interior.

Fbody has a large hump in the floor for the converter on the passenger side.

I'm wondering how different the frames are, to use A body seat covers on them.

71 A body would have had plastic seat backs and subsequently the upholstery would not cover the back. Later A body (73 up) seat backs were all upholstered. 75 got the hump for the cat in the floor, making one of the driver's seat tracks lower than the other.
Not sure on your F body buckets, I doubt 71 A body upholstery would work...post some pics as I have a set of 71 A body seats for a project.

This style seat uses the same back as a 71-5 b-body seat, just different foam