I say lucky you, 340 is much better than a 360. Did you check the side of the block to verify?
Torqueflites are one of the easiest transmissions to rebuild. There are many books out there on rebuilding them. My opinion is that transmission probably has some damage since it was run low on fluid (for how long??) and the lack of KD linkage. Unfortunately it will require more than just dropping the pan to know the extent of the damage.