I need some general info on a 420a Cylinder Head? A company I took my car to, to get the Engine replaced, and feel free to correct me if I am wrong, they took a factory Head and Deck it out at 40 over. If that is correct. Now if I'm correct Factory heads are decked at 20? Is it bad to put the car back together with it deck out that way or should I get a new head? Please help understand this. I know enough about the car just not that area.

Thanks