Keeping your head directly over the ball during the backswing causes you to shift your weight forward, instead of loading up on your back leg, causing a slice or hook
Regardless of trajectory, always use the same stance, setup and grip; just change your ball trajectory and swing length
Many golfers have long backswings where their arms keep going, keep in mind that your arms should stop swinging when your body stops turning, shortening your backswing will increase the radius of your swing and actually give you more distance