The USBInjector kext from my files are hardcoded to MBP8,2. If you want to use MBP8,1 as Hervé suggested, you should use the USBInjector from his E6220/E6420 guide which I believe is hardcoded to MBP8,1.
FYI, for the debug to be completed successfully, you must first boot to Clover, press F2 and then F4. Then continue onto OS X and then run the debug again. Do not interrupt the process while it's running. Of course you'll need a working internet connection too.