If you plan on turboing I personally would just swap the motor. If you do swap the head you will need to drop the compression because you will be running around a 10.5:1 compression ratio which we all know is definately pushing the limits when you plan on boosting, versus the modest 9.2:1 stock ratio of the z6/ With that head swap I am almost positive (don't quote me on this) but you must use the y8/z6 3 layer head gasket which is about one of the thickest head gaskets around. Which leaves you with decking the head to drop the compression ratio to avoid detonation. If you just swap the motor you have the benefit of not having to mix and match parts which helps when buying the parts (trust me I just built a b7/y8 mini-me), I have seen reasonably priced z6 swaps around and I am sure you could find one, spend some time to redo the gaskets, do a compression and leak down test, and whatever else you see fit before installing the motor then the only thing is wiring vtec and finding the z6 ecu. All in all it is far less hassle to swap the motor in and to be honest if I could go back and just spend the $500+ I dropped into my motor on a complete y8 with the same result I would.