This calculator might help you decide as well. I know it may not be 100 percent accurate as all tires tend to vary when it comes to size depending on the brand. Post a pic of the rear to get an idea of how stretched the tire is now with the current size you're running and that will help a lot. Here's a link to the calculator as well.
https://tiresize.com/calculator/
If your offset would work in the rear with a 315, this would be the perfect setup in my opinion. Rear would look amazing.
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