A photo would be cool for your post... Hard to figure out what you mean.
Sounds like it would affect your front alignment, if not more. When you get to seeking out structural parts you need to find a junker you can inspect and piece out, maybe at car-part.com. They won't list structural but you can use the sort-by-distance from your zip code to locate a dealer with a Z31 on his yard and go there to pick out what you need. Just search for a door or a hood. Non-running Z31s are gradually disappearing from the planet so don't neglect the recyclers.
By the way, you don't let on where you are - or where that Z spent its life so far. If you have rust/rot (they aren't even close to being the same thing, you know) that bad in the under structure, you should check the rest of the body on the underside to see what else may be beyond scraping and painting) Get it on a lift and go through it with good lighting, a hammer and a screw driver. Whack, whack. You may have more work to do than you think.