The LA Motor "Block"

Does anyone have a short answe on why they went external balance on the 360" that makes it a PITA fir interchangeability. i did know a guy who had a 340 manual and a 360 auto pair of setups for his dart that he drag raced in the early 80s. me, persoanlly i keep a 318 laying around and I have two 340s,but i have the parts to interchange them into the car for an engine swap just in case.