Sorry if you have already figured everything out but i just came across this post. 5.5" backspacing for a thirdgen. I'm pretty sure on that. It's been a while since I got my wheels but i have 15x9" wheels with 265/50-15 and i still have about an inch in both directions. 17x9.5" will fit just fine. I've seen zr-1 wheels which are 17x11" and 315's on thirds. Light massaging with very large hammer is needed though.