Hopefully the front seal will fix your problem. I would suggest if you do have to rebuild the Hydramatic it will likely be far more expensive than to replace it with anything else, simply because it has now become very rare and is somewhat obsolete by conventional standards. Many shops will not be experienced in working on it. I’d do a lot of shopping around before committing to a rebuild. Just because a shop agrees to rebuild it doesn’t necessarily mean you’ll be happy with the result. Also parts may be rare, hard to find, and expensive. Here’s an article explaining the complexity of the design.
http://en.wikipedia.org/wiki/Hydramatic
This doesn’t mean you shouldn’t rebuild it. The Hydramatic was an excellent transmission in its day, and bolted up to a GMC V6, the combination is rare, powerful, and unique if you can afford it.
Also - can you determine what type of fluid is leaking? The engine could be leaking from the rear main seal and drip from the same location.
If you don’t get enough answers on what transmissions will swap, I’d suggest trying the Jolly GMC website. Members are dedicated to restoration and preservation of the V6 engines and stock vehicles:
http://www.6066gmcguy.org/
You might want to try these guys for a seal:
http://www.autotran.us/hmparts1.html