I originally thought that the far storm sewer drain at the junction of the end of the picket fence and the concrete bridge abutment (triple overpass) was the most probable place for a GK shooter.
There are photos that show that cars were parked all around this area which could conceal a shooter in a ground level posture , possibly even while he was standing IN the sewer pipe!
But the problem became the angle of the shot would be very difficult to have hit the right side of JFKs head, because of how JFK had slumped slightly towards Jackie after the Z223-225 shot.
There was coincidentally if I recall, a man who was detained very shortly after shots fired , at the other end of that storm drain that opened up to a stream. No rifle was found in the storm drain, but the man could have put the rifle in the trunk of one of the cars before he used the drain like to escape.
It would also be a way for the gunman to approach , unseen, to the GK carrying a rifle by entering the drain far back where the opening near the stream was.