Go 12"
I went thru the same thing 10 years ago... have 35x15x15.5 wide Baja claws on 8" wagon wheels. tried to picture how the different width rims would look. I cut the tire loose and spaced it out from the rotor to visualize it...
1st shot shows the 8"rim on rear and 10" on front
2nd shot shows 12" wide rim on front.
last shot is the actual 10" wide rims that I went with. hope this helps... let me know if you need other angles.