Hi, Joker:
I also have a 1975 V20. I installed this back bench seat in mine several years ago:
http://www.wellcraftv20.com/gallery/album06/H
It was a great addition, in the opinion of those who've used it. It also has enough storage space inside that I was able to add an 18 gallon tank. That gives me a lot of peace of mind when running offshore. This is the model I bought:
http://www.westmarine.com/webapp/wcs...0396&langId=-1
It fits perfectly. I cut down the aluminum arms to make the back flush with the seat, then mounted the whole thing to the deck with two stainless steel flip clamps. When I need to get access to the batteries or fuel filter, I just remove the tank, flip the clamps, and move it.