I don't know if the seat backs themselves are interchangable, but the coverings may be.
I've done similar work in other cars (not Bonnevilles) and presume the procedure should be similar. On most seats, the seat back covering is pulled and streched over the frame and foam and then held in place at the bottom (underside) with hog rings. You will need hog ring pliers and hog rings to reinstall the cover(*). There is often a flap that covers the rings so you don't normally see them. You simply clip them with a good pair of side cutters to remove them, then work the covering up and off. You usually have to undo the trim around the head rest as well.
Once the covering is off the seat, you can repair it. Big needles can be worked through leather with a good pair of pliers to re-do the seams, or you can patch in new pieces of leather from a junkyard donor, or from a purse you buy at the Goodwill store. The foam underneath should be examined and repaired if it has eroded away to any great extent. You can purchase the proper thread to use and the proper density foam from an upholstery shop, or maybe get them to give you some scraps. You simply cut out the section to be replaced with a double-blade electric carving knife like you use on the turkey at Thanksgiving. Then shape the replacement foam in the same way, and insert it into the hole. If you undercut the sides of the hole, the foam will stay in by itself with no adhesive. The final contour should be flush with the surrounding undamaged areas. Bad work here will ultimately be reflected in the surface of the seat covering.
This is tedious work, but the payoff is great when you put it all back together.
Good luck with it.