Use Google to look up "Z400 spare parts", and the HP link to that will tell you:
"
Heat sink assembly - For high performance CPU | 538042-00 |
That is the Spares P/N.... you also can dig around the internet and eBay and find the Assembly P/N..
Your new processor is 130W Max TDP, and HP generally wants the workstations to go to the "Performance" fan/heatsink at 120 and above.
Side off is not necessarily better.... HP designed the best airflow for cooling with case cover on.
Also, use the forum's search bar above to specifically look for Z400 heatsink, and do your homework from that.... the answers are all there, including the assembly part number which is easier to find for purchase on eBay.