Well, except for hardware independance, high availability, resiliency and manageability or you have to run a few guest which together would take all resources of 1 host.
In case of the last situation i'd run n+1, which you cluster seems to be.
If you'd want to use all resources at once, esxi or vshpere ( or basically any hypervisor) is the wrong solution for you
As long as I know its possible, I will attempt myself soon.
If possible can you try to link the thread here ?
As my attempt would involves stripping out the chassis, then seeing how many components i can remove whilst still keeing it running (or which components can be modified) & then AFTER that, try and get the atx PSU running
(Is it best to do the PSU last, as there will be less components needing power = less modified connections on the atx psu?)
Or if someone has a better method I should try.. let me know !