Personally, I would never put 18" wheels on an '80s car that came with 15s and 16's from the factory.
First off, you'd be de-engineering the car's suspension setting, secondly, unless these wheels are very light, and they don't seem to be, you'd be making the car slower in every respect, and third, they'd look like s#!t.
Haha, that was a good one dude! But you see, I don't ride a school bus, I have three Zs to take me where I want to go. That's right, an 86 Turbo, a '90 N/A and a '90 Twin Turbo, and not one of them rides on factory wheels.
Also, I'm not here trying to force my opinions or taste on anyboby, that's why I started with "personally." But you're right, Mike didn't ask, so I should have kept them to myself.
For your information I do believe that everyone should do with their cars whatever they want, and I do happen to think that you have one of the best Z31s I've seen. In fact, I had done to my Z31 some of the things that you've done to your car, and I plan on doing others, like your fuel pump installation, when I get the cell in place. Z31 is a track only car.
But I stand by my earlier "unrequested" comment. Not all wheels look good on all cars, and most big wheel upgrades are a mistake from a performance point, for reasons I'm sure you already know. ;-)