When patterning the front of the seat back and the seat bottom, the backing for the stitching that holds the pleats has to fit the seat frame. Once that is the correct size, you can do anything you want to the seat covers. Is there foam on the seats, or is it just cotton? If it's cotton, I would replace it with foam. You will get a much better finished product that way.
No one lives forever, the trick is creating something that will.