To the OP..... good luck. There is a "Mainstream" heatsink/fan for the Z600 and these were common and thus relatively cheap used off eBay. And then there also is a Z600 "Performance" heatsink/fan which can be used with all levels of the processors that are certified by HP for the Z600. These are uncommon, and expensive used off eBay.
The HP Z600 motherboard is a tight custom designed space..... there are two sockets, and one of the two requires the Performance heatsink/fan built for HP by Foxconn. The other socket can accept that also, or a "Performance" level heatsink/fan that was also built by Foxconn for HP for the single socket in the Z400.... to make ithat fit over one of the two Z600 sockets you only need to bend down the thin rear metal airflow deflector a bit.
For the other socket you really need a Z600 Performance heatsink/fan. Why do I know? I have a Version 2 Z400 and a Version 2 Z600 and bought two spare of the Performance Z400 fans partly to see if they'd fit in the Z600. They are the same socket, after all. So, that was my discovery. The fans for the Z600 Performance and the Z400 Performance heatsink/fans are wired identically, by the way. That is the only way the motherboard knows what is attached to those processor cooler headers.
My advice: buy one of the Z600 HP Performance heatsink/fans used off eBay if you can find one, and buy another Z400 HP Performance heatsink/fan used for cheaper and bend down that air deflector. Once you take a look it is clear what socket can accept which.