Cutting the slot in the top of the castings would not be that difficult. The old style castings do not have any raised edges for the cam plate to slide on. How did you solve that?
Sorry for the late response.
When I did it the first time I had never seen the inside of the factory adjustable arms. When I installed the plate and tightened the bolt, I relized that it needed a spacer of some sort, so I just stacked a few SS washers until it was standing proud enough to engage in the cam.
It worked perfectly and it was only when I bought a newer boat that I saw the cast-in rails inside the arm.