The gains from a y8 head on a z6 if there are any would be negligible. From what I remember, the y8 IM flows differently than the z6 making better power on the lower end and sacrificing top end...that is if IIRC, it's been a while. I might have it reversed. Also, I believe you're supposed to use the hardware from the corresponding head i.e. bolts/gasket. That being said, unless you already have all the parts, you'd just be wasting your time and money for gains that may or may not be there.
If you're dealing with a shot z6 head then maybe getting a y8 head to replace would be an easier find but if not then I would stick with the z6, it's a good little motor.