Hi Robert,
Cast iron is easy to mill ! Use a slot drill a mm or so smaller so you can clean the slot width up later. Take material out in say half or 1 mm steps.
The only caveat is that sometimes an un-machined surface can have a very hard skin.
The trick here is to make the cut depth deeper than the skin. The same applies the other way, you will be cutting down to a hard skin. So the last cut needs to get below it. What I have just said assumes that you will start cutting from the already machined side of the base. Also remember that the edges will have a very hard skin, since they are un-machined.