I am in the processing of installing a brand new wall mount faucet in an older cast iron bath sink but I'm finding that the center faucet hole in the sink isn't quite large enough for both the spout and the push rod. How can I expand the hole or drill a new one to make room for both parts without damaging my cast iron sink?

This one sounds interesting. Show us a picture. How much do you need to enlarge the hole?

As you may already know a wall mount fixture is not designed to fit on a cast iron tub. You may have other problems after you get that hole enlarged. Have you checked to see if the escutcheon plate will mount on the valve after adding the thickness of the tub, the valve stem may not be long enough to reach through the plate. Do not chisel the cast tub but you can cut it with a SawsAll and a bimetal blade.