Keep in mind that the 904 converter has two different sizes of pilot (the part that sits into the crank) The early (L1962 to 1967) had a pilot size of 1.550 inches. There were also some (unsure of the application as my catalog doesn't give me this information) 1968-1972 that also had this small pilot. From 1968 - on with the 225-318-360" engines all had the large pilot size of 1.810". Your 273 might have the small diameter hole in the crank, while the 340 will have the larger crank hole. Something to watch for.
To the best of my knowledge, ALL 340's came from the factory with the 727, not the 904. The 71 Duster I bought new had the 727 in it, as did any other automatic equipped 340 I've looked at.
Hope this helps.