With the positive offset of the wheels you should be able to fit 315/70/17. Thats what I am running with my two inch level. Here are a couple pics.

 

How does the offset work? My wheels stick out about 3" past the truck. I was told they look like atleast -12, so negative numbers push the wheels out further?

 

Positive is how far the mounting part of the rim is pushed outward and negative is opposite being that ur wheels will have the mounting part towards the back of the rim pushing the rim and tire out. I have a -12 as well