Less backspacing (lower number) will push the tire out farther. Same thing with offset, the lower the number the more it will stick out but you want to make sure it is a negative number. Spacers I'm not really a fan of but it will allow you to keep the wheels you have now.
IMO tire width doesn't have nearly as much affect as rim width. A 12" wide rim on your 315 tire will give a fatter look than a 325 or 355 tire on a narrow 9" rim.
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