I have read the post about tire sizes but I think bigger tires will fit. I have a 2003 F150 ext cab and I have 285/70/17 on it with no rubbing and I actually think I could fit the 315s from a hummer on it. I do have the reindexed keyways on it that all the chevy guys run and yes they do work a lot better then just cranking up your T bars. I also replaced the 1.5 inch block in the back with a 3 inch. Let me know you thoughts.

 

285/70r17's should fit any F-150 with out rubbing.

Depending on the width the 315s may fit with your mods... but it will be the width that's important....most times this is where the rubbing comes from.