I need to fill my center mount dipstick hole. I was gonna tap the hole and insert a small plug plug. I've tapped lots of mild steel but how hard is it to tap the cast iron block?
Threading the "freeze" plugs is not uncommon. A few high end engine builders do it.
If an engine block is gonna crack from freezing, then it's gonna crack. Those plugs aren't really designed to save an engine from cracking. Sometimes they do, sometimes they don't. They're only there as part of the casting process.
Old BOSS 302 blocks had this done from the factory I believe.