Align the Body
Sit the body on the posts and align it from front to back, making sure you get the center of the body in line with the center of the truck’s wheelbase. Or, you can mark the center of each wheel well and align those up with the centers of the wheels. If your truck’s wheelbase is slightly longer or shorter than the distance between fender openings, just eyeball it until you find the position that looks best and minimizes the difference.
Now, look down at the body and align it from side to side. Do the best you can by eye, and then use a ruler or pair of calipers to center the body over the truck’s body posts. For example, measure from the center of the body post to the edge of the hood, and move the body until both posts have the same measurement.