Not helpful if you're comparing it to a "pencil neck".
It is not the length of the neck that is wrong, in Fuhrman's drawing. Rather it is the position of the exit wound which was in the nape of the neck to the left of JFK's tie knot. Your profile photo shows that Fuhrman's placement of the exit point of the bullet is nowhere near the actual exit wound on JFK (which is below the bottom of the photo where his tie knot would be).