Milling wide sections of cast iron can be a real chore- and especially if a decent flatness is sought. I dulled at least two shell mills and one slab cutter milling a casting to be a scraping practice piece. This is where a tool and cutter grinder would really come in handy.
I wish I could have found a hunk of cast iron to harvest here recently when I had to order a 3 inch square piece on line. 3 inch square cast iron cost me $72 delivered to my door. No joke. 😱
That's if you can get it. Tricky to source here in the UK , and premium prices too. Having said that the stuff I am using can have voids and inclusions which are a pain all best