I just went through this and you will find a few different answers. If you are going to be running a 17x10.5 in the rear, you WILL NEED to get the button head MM bolt for the IRS and 3/8" spacers. I just got done going through this whole fiasco.
If you are running 18's in the back, you should be fine. That IRS subframe bolt is your enemy.
As far as the fronts, you can go pretty wide. I am running a 255 in the front, but the 03 came with 275's and if you are running a 9" wheel up front, 275's will fit nicely.