Well these are the OEM 343M wheels for M6:
Front: 9,5JX20 ET:31
Rear: 10,5JX20 ET:19
You have to know the new rims ET and then you can use this online calculator:
http://www.1010tires.com/Tools/Wheel-Offset-Calculator#
As far as tires, for 10.5x21 rims you can go up to 305 (what I am running right now)
so basically, 295/25/21 or 305/25/21.
For 9x21 rims the widest tire you can go with is 275; so 265/30/21 or 275/30/21 are your options.
I highly recommend MPSS tires... they are really fantastic