Hi Erik,
The 54 and newer left case wil mate with the 52-53 right side case but some machining is required.
The front picture is a modified 53BN that was mated to a 56 left side case. Right behind it is a 52BN case and in the far back is a 55BN case. you can see on the 53BN case to mate to the 56 case relief had to be cut for the stroker flywheels and the rear case bolt is repositioned.