Wheels and Spacers
 

On a 1988 convertible, will 17x9.5 ZR1 replica wheels (56mm offset) fit all around without spacers?

This is the wheel. https://www.jegs.com/i/OE-Wheels/494/4750784/10002/-1

Take a look at the back of your existing rims, I believe the backspacing is likely cast into the rims. I don't know when Chevy went to 56mm backspacing for sure, whether it was in 88 or not. I know they won't fit on an 86 properly, that you need to get 40's.