Valve pocket/bowl hog tools....

You can buy bowl hogs from Goodson and use them in a drill press if you can hold the head and level off the guides.

No freaking way would I do a plunge cut with a stone.

Naw I ain't doin all that. I thought about it. The cutter(s) I'm lookin at have pilots that use the guides to center them up. That makes it pretty stupid proof. I need stupid proof.