The 70 rear end could work. It will probably require some cutting & welding on the spring pads. Also be aware that the wheel bolt pattern will be different. 6 lug vs 5 lug.
The small block with 350 auto combo is a very common switch for these trucks. You should be able to find lots of info on the conversion.

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