I have the original Intel Mac Pro 1.1 and I know it has a 64-bit CPU (Xeon), yet it has 32-bit firmware. However, I know that it's possible to patch it to run 64-bit OS, like OS X Mavericks (which I used to run on it via SFOTT).
I now run VMware ESXi 6.0.0 on the machine so I can run VMs. ESXi 6.0 is the last version officially support on the Mac Pro 1.1, but just like OS X Lion -> Mtn Lion, I am reading that it is indeed possible to run VMware ESXi 6.5 on the machine.
My question is this: Do I need to "patch" the firmware of the Mac Pro 1.1, or simply the ISO installer of the OS? When I installed Mac OS X Mavericks I simply patched the ISO. I found this post about installing ESXi 6.0 on a Mac Pro 1.1 and in the comments the author says he thinks the process is the same for ESXi 6.5, but is not sure.