Proper rocker arm geometry creates the thin line......there are guys that can put it into words better than I can, but I was always taught, it is the trunnion height. When you set up shafts, you set the trunion height, and then measure the pushrod. Pushrod length is an outcome of proper stand height.
I remember when talking to Bill, or Don, I didn't think that the shafts could really be that much better. They were right, there is no comparison. Money well spent, will use shafts, in my wifey's motor too.