It depends on the block... The cast datums will be called out on the blueprint drawing for the block and also referenced on the block machining blueprint... Those drawings are not usually shared with people outside the process...
On our v-6 block our cast datums for the machining were inside the outer cup plugs on the end cup plugs on each side... They were flat pads about 3/8" to 1/2" diameter where we had "fingers" on the machine line that came in and rested on each pad to set the location and find the center... That's what we used to set the location of the manufacturing hole which was our datum for all further machining after that...
All operations after that used a pin that fit in the manufacturing hole (tight fit) to locate the block for further machining... Bores, crank center line, etc....
Maybe if you ask the place that did the block work, they may help you....
