John,
That boot block date is for the version 1 of the Z420 motherboard. If your boot block date was 3/6/13 you'd have the version 2.
Next step is to gather up some copies of the Z420 Quickspecs. They are pretty easy to find with Bing and Google. HP even has somewhere a repository from which you supposedly can get every version. I have v 2, 27, 32, and 41. As you go through the versions you can see the transition point where the Quickspecs start to focus on the new version, with that later Boot Block Date. That transition is generally not stated but you can see from the processors listed when it happens.
You're pretty much stuck with the processors from those earlier Quickspecs. HP has a segment of their BIOS that deals with what processors are allowed to boot, called the microcode. This is to keep the processors limited to the ones that they have certified for quality assurance. HP does not "support" processor upgrades like you are planning to do (and which many of us have done). But, it can be done if you stick to the processors that are in the Quickspecs from the same era your workstation is from. The microcode is generally updated with a BIOS update, and that is one reason to update BIOS to the latest. A good rule to know is that just because it should be possible does not mean it will be. The microcode generally only allows the very specific processors listed in the Quickspecs to run. I have never seen a new microcode version delete a previously allowed processor. The old processors will still work even though they may not be listed anymore in the new Quickspecs.
I only install used processors bought off eBay. I research the sSpec codes of each processor and if there are more than one for a processor I always use the most recent version. I only use Noctua thermal paste, quite thin layer, to the degree you just barely cannot see through it. Once I know the processor I want I'll search for it both by name and sSpec code for best buy. Very rare to get a bad one. Never have gotten stuck with a bad one, ever, because the seller will refund or send another.
From all I am seeing your desire to use a v2 or above is off the table......
You could consider a motherboard transplant, however, to get a more recent one installed with the later boot block date. Then you could use processors from that v41 Quickspec I mentioned above. Google for Z420 spare parts..... you'll see a HP page with that title, and in there you'll see 2 version 2 motherboards and one version 1. You can tell the newer from the older one ("Patsburg" is hinted at for it). Either of those newer ones that start with 7 would be fine if you want to take on that project. It's a pretty easy afternoon project.... search on eBay for a used one if you wish.
Best, Scott