REPLACING HATCH GAS SPRINGS
Step 1 - Removing the Old Springs
I did not take any pictures for this step as it is not very difficult
DO NOT TRY TO POP THE FACTORY GAS SPRINGS OUT. The ball mounts are cast directly in to the body of the spring and must be cut out. They will not give if you attempt to pry them out and you'll be left with a stripped mounting hole in the body of your car.
I suggest supporting the hatch with a 2x4 before removing the springs because it is rather heavy once they are removed.
Step 2 - Freeing the Ball Mounts
On later models of the Z31, Nissan decided to have one end of the hatch springs be supported by a stud welded to the body rather than a threaded hole.
I had no luck finding this super weird ball mount with a nut anywhere online so I decided to use the ones provided with the OEM springs.
I recommend doing this by hand with a hack saw to avoid scoring the mount. The housing of the spring is aluminum so it is easy to cut through. Proceed slowly and check your progress frequently. The ball mount is a much harder material so it is obvious once you have made it all the way through the spring body.
Step 3 - Installation
Place the mounting hardware back on the body of the car. It would not hurt to include some type of gasket to prevent water ingress.
Once they are securely screwed on, push your new spring directly on to the mounts and enjoy your hatch that can now support its own weight!