That's a pretty good crack. I've always had cracked valve seats but like Josh said, not that thick . Kind of the nature of the head if it sees knock. Some of the castings just down right suck to begin with. It's not that the casting has gone bad, but it was made crappy from square 1.
I have fun with bad castings. There's an inside view shot of one of my recent intake runners on my page. That head had 30 thousandths of core shift through the entire intake runner.