I too suspect the max memory possible on the z420–at least using rdimms—would require mixing two different dimm sizes on each channel. Using lrdimms I think 512gb is possible on e5-2600 v2 series with 8x64gb modules.Yeah, there's a certain point where motherboards will wear. Luckily I haven't had any issues so far, but I also haven't pushed it at all like I originally planned.
I'm pretty curious too as the machine wasn't even supposed to support RDIMMs or LRDIMMs at all.
I personally think you should buy a single module and see if it boots at all. If it does then throw in your existing 32gb dimms and check it still boots. If it does buy one more 64gb module and see if it boots with 320gb. If it does I’d say you’re almost certain to succeed with 512gb booting.
doing it that way doesn’t waste any more than $40.