Biggest rear wheels and tires

You're probably not going to be able to get anything much bigger than 325's out back. That's what I have and it's tight. That's with 15X10 wheels. I have my Billet Specialties wheels (15X10 and 15X4) for sale in the tires and wheels section if you're interested. You can make them work with the backspacing since you are narrowing your rear already.