[blakjak55]

You don't have to swap the whole diff.

GM Kit comes with a recal authorization code that they'll call and get the data to change your speedo and odo.

If GM does the work your warranty is fine. You're going to pay for peace of mind. Doing it yourself is cheaper, but harder and will not be covered unless you find a mod friendly dealer. Having the dealer do it will cost more but no worries on the back end.

American Axle and Machine (AAM) makes the GM gears for the factory and for the kits.

3.91's work perfect for an M6 until you go FI. After that, you may want to go back to the 3.45's. 3.91's make 1st a hard launch and 6th finally usable.

I agree with SS376; ask about a tech who's qualified and has set up gears before. If they have one, chances are he knows how to do the recal also.