EFI Boot Frozen When Trying to Install Windows 8.1
If you want to EFI boot the installer you need to flash your firmware from MacPro4,1 to MacPro5,1.

Create an installer like mentioned here without any bootmgr files to force an EFI boot.

Remove all disks from Mac Pro to avoid further trouble with Mac EFI looking for MBR or protected MBR.

EFI boot your installer.

Be sure to clean the unneeded Bootcamp partition and create GPT install target.

See also here.