If your new SM420 has a driveshaft parking brake it came out of a big truck, the 1/2 ton units don't have a brake on the rear, they use the rear axle parking brake.
The drive shafts are not the same between 1957 3-speed and 4-speed, the 3-speed has the driveshaft slip joint as part of the transmission tail shaft (yoke slides into the trans with U-joints on both ends of the shaft) while the 4-speed has the yoke flange bolted to the trans output shaft and the slip joint as part of the driveshaft (shaft has U-joint at rear and slip splines on the front end).
Mike B
