You could go with a reverse grip and go down to 66". If your Robertson has some shape to the grip, like mine do, even though the Hill grip can be deeper, the change in your grip can slightly effect your draw length. You may also find that a Hill will require a softer spine pound for pound, although with a quicker Hill style bow with a slight bit deeper arrow shelf, they will be very close to the same.
My son also shoots Robertsons, he went from Hills to a Pete George to a McDonnald then to Robertsons. I was in the process of buying him one of the new Schulz bamboos, he absolutely did not want one, nor does he want one of the new Hills.