Hi new here and I tried to install via usb on a EFI machine (macpro 3,1)
Since this is Ubuntu based, it should boot since an Ubuntu USB has no problem booting.
Another question is why did you choose Ubuntu over Debian to base the OS off of?
In theory it should work, however that is a 17 year old machine so you may run into issues. Id suggest getting something a bit more modern so that it will play nicely with modern OS and modern gpus too. 17 years in pc terms is a lifetime.