Lost Frost plugs

When they "ran it in" with an electric motor, even though there is no combustion taking place it would still generate heat from friction - and I am betting that they had some water in the cooling passages for this reason, to avoid overheating the rings and taking the temper out of them. Unless you removed the pipe plug drains (one per side just above the oil pan rail) on either side of the block any water that may have been in the block would not drain out, as the feed holes to the water pump housing are higher than the level of the block drains and core (frost) plugs. I would be pulling that engine, and looking very closely along the sides of the block for hairline fractures. There is no other possible explanation for why the plugs were pushed out, other than freezing. I have seen more than one block ruined by having water remaining in the bottom of the cooling passages.

Look at the other plugs, and see how many of them are partially pushed out as well.

A good friend of mine with a 440 in a race car Duster only ran water for coolant. He thought he drained it all in the fall by removing the lower rad hose, and then it sat all winter and cracked the block behind the starter.

Have a very good look, it is easier to find out now rather than later!