Don, TH350's were the rage in the early eighties. Mid to late eighties saw the migration to the O.D. trans. If a 350 is what you want I doubt you will find a "nice" usable trans in a junkyard. You would be much farther ahead to simply go with a 700R4 or just buy a TH350 from a good rebuilder. I know it will cost more up front but trying to save money by buying "junk" willl only cost more money and time in the long run. Spend it once... Spend it wisely...
Mark