Ok, I think your fuel injectors are leaking and you are trying to fix it buy buying new seals and insulators. That will not prevent them from leaking. You need to buy new injectors if the injector itself is leaking fuel. The lower rubber ring seals the injector to the engine. Besides that, you can have a cracked plastic insulator (older 280Z's have aluminum) and they will be fine as long as the injectors are ok.

My plastic insulators cracked when I gently screwed the injetors in. I pulled the whole fuel rail and replced them with another set from Nissan and they cracked again! (this is while I'm installing NEW fuel injectors) so I decided to leave it alone and see what happens. 3 years later and 50,000 miles have gone by and no fuel leaks. That plastic sleve is just a holder. It is not holding fuel pressure.