The main problem once the body is painted is making sure you get it positioned properly. If the paint is dark or heavily applied, the flashlight trick may not work.
I always trim the body posts near to there final length first. Next I plop the body on and stand over the car. I then align it fore and aft then side to side. once it's in a good position, I hold the body down and crouch down so I'm looking up at the wheel wells. It helps if the car is perched near the edge of a table and placed on a car-stand.
Taking a Sharpie marker I then mark an outline of the body posts where they make contact with the body.
Then you drill and mount.