Do a search on the HO52 and HO72 rear ends. There is a lot of info here. The only gear set available for the corporate axle is a 4:10 from the 68-72 C-20's with an auto trans. Probably not much luck finding a R&P, likely need to find a used assembly.
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