It's the factory rear end. That era 3/4 & 1 ton has a partial torque tube from the trans to a cross member and a flange that connected to an open drive line type rear end. There is an optional rear end available which is from a 67-72 truck with an auto trans. It has a 4:10 ratio. They are getting difficult to find however. There are numerous posts about this subject on this site.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine