Leaking transmission

My 65 Dart GT slant 6 904 auto is leaking. I backed into the garage today to do a little work on the rear wheels, when I pulled out I found a good puddle of tranny fluid and checked it was coming out of the bell housing.
My issue is: do I just pull and replace the seal? Or what I am more apt to do is to pull it and have it rebuilt (I'm at 123K) . Or the nuclear option of pull it all and find a 273 engine and tranny and just upgrade.
What is the basic cost of a bench rebuild at a tranny shop?
Are there any major changes needed for a V8 swap?