First off, WELCOME! A lot of use are hovering around the half century mark so you should feel right at home.
Your choice for an engine conversion project sounds like fun. I, too, have been digging into the 4.9 swap topic and Bubbajoexx has done us all a big favor with his research. He may also be the one to ask about the stock 4.9 tranny.
There is, however, a lot in the archives on automatic tranny swaps. Most are going for the 4 speed auto to gain the overdrive function. 440T, 4T60(?) seem to be the simplest. Do a search on "transmission swaps" or "overdrive" or even something like "440T" and see if some of the old posts can help.
Hope this helps,
Roy