Because the 67 and earlier cranks had the smaller register. I am not sure about 68 and 69 cranks , but my 273 had the smaller register in it. I found out the hard way, after building a 727 and getting a torque converter, then realizing the delema.
340mopar said
How is it then that I have a '69 273 out of a Fury that had a 904 that is now in an '85 pickup with a 727?