Tutorials? Plenty of them!
Plenty on the web showing step by step how to upgrade the 4,1 Mac Pro to accept 6-core goodness.
I have done it and its pretty easy. If you ever upgraded a processor before on a PC, you should have no problem at all. Remember, VERY IMPORTANT: If you use the 4,1 to 5,1 firmware understand that if you have the w3680, or 6-core Westmere chip in place and revert back to 4,1 firmware, your machine will NOT boot as 4,1's firmware does not recognize Westmere, or B1 stepping.
I would seriously and strongly consider you flash EFI firmware to 5,1 and get the 6-core and don't look back to 4,1 unless your machine is under AppleCare. Then you would have to remove the 6-core processor, re-flash the firmware back to 4,1 and then Apple can warranty repairs for your machine.
Another note: If your 4,1 Mac Pro has B08 firmware, I have the B08 should you need to revert back to it, otherwise if you go from 5,1 back to 4,1 and your machine has B08, you will lose that one forever and it will go back to B07 which is the default firmware which came with the later 4,1 Mac Pros.