The most common rear in a 57 would have been the corporate rear which was 3.90. The third member will bolt right into a 54 and earlier torque tube banjo housing. An option could have been a Spicer 45 at 3.92 or a V8 with an automatic could have been a spicer 45 at 3.07. The Spicers have the carrier built into the banjo housing instead of a removable 3rd member and more closely resemble more modern rears.