A little progress after the new chunk of cast iron arrived. Pic 1 Nominally it is 130 x 60, but is slightly hollow on the width.
Pic2 shows the recess being rough cut with a 20mm Little Hogger, after marking out.
I deliberated for quite a while as to whether to drill the stud hole first or last, and decided to drill it first.
The stud was locked into the top-slide by a hex-keyed screw from underneath, so was easy to remove.
Pic3. The recess was finished to size with a 6mm carbide 2 flute, usually used on my router, and it worked well. Also used to finish the whole depth to size.
The part-finished chunk fits the top slide like a glove. I would like to say that the stud just dropped in...but cannot.
After using a 1/2" drill to enlarge the 12mm hole..it now fits, as pic4.