All '66 Darts with an automatic had the 904 transmission from the factory. Yours could have been changed sometime in it's life however. Usually you have to "message" the firewall a bit for clearence but it will fit. The pan on a 904 is shaped like a square with one cornre cut off. The 727 has an irregular shape with a bulge in the pan around the dip stick tube area. It also uses a 29/30 spline output shaft instead of the 25/26 version and most 727's have a longer output shaft and extension housing.
