My bet is your gonna have to make some adjustments still
Awesome, I plan on running the +18 since I have the OEM fender flares and want it to fill them.You will have absolutely zero issues running the 285/65/20s on a 20 with that setup, just be sure your backspacing on the 20" wheel is +18 or greater. 3" OME with a 3" CB Shackle in the back should give you about 1/2" rake rear to front.
You will probably have to bend the edges of your skid plate up 90degrees and maybe remove the front mud flap. Otherwise they should fit just fine and look good.
I'm running 3" spacer front, with 3" CB shackle rear and 275/65/20 on a +60 20" wheel.