If the transmission is a 727, it may in fact be a big block transmission, I'm not familiar with the difference between the big and smallblock trannies, But I believe the starter location is different somehow too.
i would like to see a slant 6 727.i believe they were never compatable or produced for the slant 6. the 904 is 31 inches long and the 727 is 35 inches long regardless if it is a big or small block. the way to tell a BB 727 from a SB is the size of the converter housing. the starter is in the same spot on both. use the link i gave you to determine what you have. the spline count alone will help