340-904 thrust bearing/crank issue

Hello to all, and thank you in advance for reading my post. I am new to this site and will do my best to describe my dilemma. I recently purchased a 67 Dart GT. It has a 340 coupled to a 904 that has been modded to reverse pattern ratchet shift and 3800 stall converter. The crank is ruined at the thrust bearing.disgust The engine is being torn down and I'm awaiting the recommendations of the machine shop as to all it will need. (so far a new crank) The transmission is the main suspect in this, it was purchased new ($1600) by the previous owner about a year and a half ago. My question is did he do something wrong with the install and what should I do to avoid having this happen again? :sad11: