Over the years it has always been SOP to replace freeze plugs whenever you do any major engine work. Almost always they have been eaten away on the back-side even when the coolant or Waterwetter was used properly. A rather cheap and easy thing to do but a major pain in the ass if the dumb things fail when the engine is together and in the chassis. A 10 min. job when assembling an engine or a weekend if you find one leaking when the wretched mess is all together. From your pictures I would be willing to bet that the plugs are gone by the way.