well ideally you would mark the spots to reamer your body mount holes before youve painted the body but if you havent then what you can do it stick some small magnets to the body posts and sit your lid down onto the body where you want it and take a couple of BBs (or another magnet) and put them on top of the body. They will stick in place where the magnet is and then you know where to ream your body post hole.
Great idea!!!! It beats the sharpie method too. I use to put some sharpie or small amount of paint on the top of the body posts, center the lid, and transfer the paint to the inside of the lid.