I have a 67 289 I don't know how similar they are. When I had to replace one of my freeze plugs this is how I remember doing it. I used a screw driver and placed it at the edge of the freeze plug and hit it with a hammer. This caused one side of the plug to go in toward the block and the other side came out. Its like flipping a coin. Then you can grab the part of the plug sticking out and pull it out. Obviously it will be a little more complicated if access to the plug is blocked by the engine mount.
Hope this helps