Apparently some on the forum have never seen the old style concave freeze plugs. They were in the early big block Mopar engines and 318s also. I remember changing them but I've had so many Mopars since 1956 when I got my first car that I don't remember which engine it was. Anyway, for everyone's information, here's a pic of what they looked like. It's not a very good pic but you can see the step where the freeze plug goes. This is part of a pic of an early big block engine.
You set the freeze plug in with the high side out and then you hit it in the center with a hammer to expand it. The cup style plugs will not work in this type block.