Hi George, I don't know of anyone that reproduces that channel. You will need to get a good used one and weld it in. I had the same type of rot on my 65GS project and i was able to get a good right side three years ago from Mark in TX, but he recently retired and crushed whatever he had in his yard. I would give Desert Valley Auto Parts in AZ a call (800-905-8024) and see what they have to offer and how much it could cost. You can give James Kehr in PA a call too (215-767-0845) he covers 1959-66 Buick's . Also there are other scrapyards throughout the country, just look in Hemmings Motor News and check out there inventory on the web or just call them. If you run into a dead end I'm sure one of us could come up with something. Let us know..
Tony