Kept out of this up until now as I initially did not know why this modification was being done. The machining is a very simple procedure using a slot drill or end mill in a vertical milling machine with the work piece held on a rotary table in dividing head mode. If you do not need slots for ongoing adjustment, just for a one off movement, why not weld up the original hole and mill a new hole (slot drill) in the required position for changing the angle?

