I'm not talking about the shackles, their mounts, the spring eye bolt and it's mount.
Your 51 originally had leaf springs with a pin that goes through the leaf pack and the end of the same pin fits a hole in the bracket welded to the rear axle. This pin helps keep the spring pack in place. With a donor rear axle, you typically dump the original design out the window along with your 1) and 2) above. Usually the later year donor axle design spring mounts don't work with the original springs so these axle parts are usually ground off and a new weld-in mounting kit is used. Some can have home brew designs (close eyes and not look at). You purchased new springs for a different generation of trucks which adds yet another complication factor.
I would go back to the spring vendor and tell them what year/model rear axle you have and ask what they recommend for installation parts or kit. If you want Stovebolt members to help, you need to identify what axle you have, what model number STL springs you have and provide pictures.
