As regards the tire portion of the question, a 205 50 tire is a squeeze on a 5.5" rim. That is as wide as I would go if I were you. I would even consider going to a 195...

As Arne pointed out, those hubcaps were available on base 280ZX's. My brother's '79 had those very same 'caps.

I will be getting 280Z rims from a '77, not alloys from a ZX. These will take the hubcaps, but as Arne mentioned, the date code is mildly problematic. However, there are no immediate plans to win a Gold Medallion, so these will work fine for the time being.

Remember that the 280Z stock rims are 5" wide as opposed to the 280ZX steel rims at 5.5" if you're still considering tires wider than 205-70 or 195-60.