If you don't put an 84 or 85 fender on it, it will look stupid.
The 86-89 fenders are flared differently and won't match the rear flares. And if you do one side and not the other, it will look even dumber.
Having said that, there are no issues with swapping any year z31 fender. They bolt right on with minimal work (some may need a hole drilled).
http://www.az-zbum.com/informa...ender