My father is the only one I'd trust to work on one, but he's 86, and doesn't do much more than offer advice when I'm into a transmission job. He fed me and three siblings all through the 1950's by fixing those things, probably a few thousand of them! He was the first or second man in Nashville to work on automatic transmissions, starting in about 1946 or 47. My experience on the original Hydramatics is pretty limited, but I've done a few in my time.
These guys are probably pretty close to you, and they could probably reccomend a good builder in the area. I've done some buisness with them, and other than talking funny, they're pretty good guys to deal with!
http://www.fatsco.com/index.htmJerry