I bought this aft bench seat from West Marine:
http://www.wellcraftv20.com/gallery/...bum06&id=H
The interior width of my V20 at the transom is 70". The width of this seat is 60". The fit is perfect, and it provides extra storage capacity as well as comfortable seating for three. I installed another 20 gallon tank under the seat, and I still have room for oil, rod holders, life preservers, and other items.
Good luck with whatever you buy.