"Prohibited" sign on boot
Try booting verbose. If it's complaining about OsxAptioDrvFix and relocation block, make sure you're booting from a drive with the latest version of Clover (3270+). This was a common problem with older versions of Clover.
If OsxAptioDrvFix doesn't work, try OsxAptioDrv2Fix (but not both at once).
For a 2500K, 2600K, or other Core i5 and Core i7 'K' Sandy Bridge CPUs you may need to boot with cpus=1 until you can either install the correct SSDT.aml or finish the install and put NullCPUPowerManagement.kext into /Library/Extensions/
If you get all garbled text and/or the message "Still waiting for root device" with the prohibited sign:
Try the install USB in every single USB port on your case, motherboard, add-on PCI card, USB hub, etc. and see if it gets farther on any of those
If you're using a USB3 stick, use a USB2 stick and try all USB ports again
Try booting with cpus=1.
If you have USB hub or a port replicator, try putting the install USB in there
Double-check SMBios selection: MacPro3,1 may be the most reliable in general