Centering front to back I used a clip to hold a plumb bob at the peak of the wheel well and lined up the front first, then the rear. There is enough forgiveness in the bed to adjust fore/aft a bit if you've got the wheel base right to start with.
Width wise you'll need to get the spacers to get the front right and the 4x4 axle is the way to make the rear come out right or you'll rub tires on the bed side's lip.
