Well it sounds like you found some use of wheel spacers to eliminate rubbing on the frame but yes, they will push the tires out past the fenders and may cause rubbing on the back end of the front wheel wells and possibly on the front bumper valance when turned.
I have no experience with OEM rims on 285 75 16 BFG KM2's but I don't suppose it will be any different from my friends recent set up with 285 75 16 Duratrac's on OEM 3-split OEMs, Spidertrax 1.25" spacers and only 2.5" of front and rear lift.
The only trimming he only had to do was on the driver's side bumper valance and none on the wheel wells. Knowing you will have atleast 3" of lift.....you may jut get away with it....if you plan on wheeling your truck, then that's a different story.