What is the largest tire I can fit in the back W/out any modifications? Thanks.

Are you talking height or width or both? On some of the 2nd gen forums, I've heard guys going with the 10" or 11" wheels with a 6" backspace but I think that is really, really pushing it.

I'd say an 8" wheel with a 4.5" backspace for the rears and a 7" wheel with a 4" backspace up front. A 255-275 is about as tall as I would get. Again the 2nd-gen guys have gone bigger, but I think you might be asking for tire rub at some point.

I often recommend folks that want to maximize their tires and wheels to purchase a tire/wheel measuring tool first, like the Percy WheelRite. It will tell how much wheel, tire, and backspace you can run on your fender wheels. Its money well spent before you drop big coin on the wheels and tires - only to find out they don't fit.

https://www.summitracing.com/parts/php-01201?rrec=true