If the 4 speed in your '57 is original, it's an sm420. Those bellhousings have a 4 11/16 bore for the front bearing retainer.
The sm465 that came along in '73 had the bigger 5 1/8 front bearing retainer.
SM465 came out in '68. As I recall early versions had the 4 11/16 retainer. Someone can chime in on that.
