Tires!
 

What's the biggest size tire that will fit on a stock 97 GMC rim? Will I run into any problems if I get the biggest size possible?

A friend said 285/75/16 will fit but it wasn't 100% sure. ??

The rear would not be a problem. I ran LT285/75R16 tires on my 91 4x4 but I had to cut the chrome trim molding on the wheel well up front and fold it up otherwise the tire hit it when off road. My 2004 GMC Z71 I run the same tire size and it's OK stock. This is basically a 33 inch tall tire with a width of about 11.25 inches. If you are buying new tires some dealers will put one on your rim and let you try it.