A good grip and solid shoulders are inextricably connected during the Overhead Squat. If you are forced to use an extremely wide grip when overhead squatting and find it challenging to get in a good overhead position even when standing at the top of the squat, your upper back, chest and shoulders may be so tight as to not allow for good overhead squat movement. Working on this point should be a top priority but it might not be enough just to stretch them.
If you are that tight, try to loosen it up by taking a lacrosse ball, tennis ball or some similar round object and lay down on it. The ball should be on the muscles between your shoulder blade and your spine. Put as much of your body weight on the ball as you can stand without wincing (you know the feeling!) and start to make snow angels. Start at the top of your traps and work your way down on both sides. If your pectorals are tight, this same process can be done by laying flat down on top of the ball starting where the pec and the deltoid meet. This active release method can do wonders for loosening up the musculature.