Any rear seat from a 64/66 will bolt right into a 63/66 wagon IF you use the Barracuda hinge brackets for the seatback. The brackets bolt right to the 3 studs coming out of wheel well. Don't forget you'll need the catch brackets that are welded to the wheelwell so the bar on the seatback will latch. That bracket also should have the plastic or fiberglass cover to finish it off. I have mocked up and it works great a rear 66 fold down seat in my 65 wagon. A plus for using it is upholstery is readily available for it unlike for the wagon.
