I've had my 1956 Cadillac Hydramatic rebuilt as well as my Ford C4.
I agree that you don't want to take it to just anybody and just because they have good reviews doesn't mean they're capable of doing a proper job on a 50 year old transmission that's been out of production since the 1980s.
Having said that, I found a local shop in Atlanta that had been in business since the 60s. The guys were all in there 60s and 70s and were familiar with the Cadillac tranny. Excellent job.
An AAMCO location 30 miles away from me was recommended to me for my C4. The guy who did the work was an expert on the old Fords.
So, I'd talk to every transmission place I could find. Ask to speak with the person who'll be doing the work. Maybe the local AAMCO has an old timer doing the builds?
Another alternative is to remove the transmission (or have it removed) and ship it to an expert.
http://www.appliedgmc.com/ might be able to help you. Call and talk to Jim. They specialize in GMC Motor Homes which utilize the Toronado engine and drive train. They are a great resource for parts and advice. They even do rebuilds.
Call and talk to Jim. He might even be able to recommend a rebuilder in your area.