To "rebuild" the strut, you need *new* parts. The the rebuild kit will have full instructions. You need to carefully measure the correct amount of the appropriate strut fluid. It will seem to soft or too hard if the wrong amount is put in.

Are you trying to rebuild it, or just swap used parts from one strut to another? If the latter, you still need a new gland packing, O-ring and damping oil.

The Datsun 1200 repair manual gives full instructions, what parts to grease, how much oil to use (depending on which brand of strut used -- there were two: Atsugi and Tokico). It also shows the order all the parts go on etc and how much to torque the gland packing nut.

I wouldn't attempt this without the full instructions.

Our local public library has Clymer, Chilton, Drake and Haynes manual for Datsun 1200. At least some of these have the strut instructions as well.