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It's an insert like all first gen struts. The big nut on the strut tube comes off. When you put the new insert in, pore some motor oil in the tube to help with cooling.
Use some penetrating oil on the nut before removing it. If you still have problems taking it off, use some heat from a torch.
Possibly someone else with a screwdriver and hammer in the other groove at the same time.
Or,
Heat the tube with a torch as KansasCityREPU said, and try the above.
Or,
Drill out the spot welds on the broken off part and weld it back on and then put some heat to the tube before torquing on it.
Use some heat. I did the same thing on two strut tubes. I tried a chisel and it didn't work either. Rust has it frozen. Heat will expand it and brake it lose hopefully.
I got a large vice grips on the threaded part and along with a large pipe over the handle for extra leverage I finally managed to bust it loose. Fortunately KYB furnished new nuts to use.