You may have found something everyone is looking for. A 4.10 was an option in 67-72 3/4 ton trucks but not in a 57.
The center section may have been changed out using a later assembly. If so, go with it.
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