I think you will need 6.5 back space. I have 9.5 wide with 6" bs on my step side. I would not want them to stick out any farther than they are. You might even want to go 7"...get a plumb bob and do some measuring. How much wider is the tread on the tire than the rim?

 

the more bs you go the more grinding that will need to be done. i have a set of 15x10 with 6" bs and i did some grinding on the caliper but i dont think there is any way to get them to fit. maybe it is the rim design or maybe it is the deep bs but i think i would have to take off 1/4" from the caliper to get it to fit. the rim barrel is smaller in the middle than it is on the edges and the more bs you have the more of the smaller part of the rim to the inside of the wheel.

 

I HAD TO TAKE OFF MORE THAN A 1/4"

MY TIRES ARE ABOUT 1.5 WIDER THAN THE RIM

I AM WORKING ON GETTING A SET MADE FROM EAGLE TO MATCH WHAT I HAVE IN 18'S