From the picture you have with the shackle missing and the spring mount against the chassis it appears the spring is to long. That mount should be closer to that position when the spring pack is almost flat not when it's not holding the weight of the car. It looks like if you had the shackle in and compressed the spring the shackle would actually fight against the spring when extends its length during compression.

On all my datsuns the rear shackle is almost vertical and is not far past lining up with the chassis mount and they all sit with the springs not far from flat with the weight of the car resting on them.

It would be my guess that they are not the springs for that car.