'68 318 - Coolant left in heads when removed.

AJ/FormS said
Another plug.

There is a screw-in pipe-plug (both sides) just above the oilpan rail, towards the forward side of the midline of the block, between the front two freeze-plugs.
The removal tool might be any of three; square socket, hex wrench, or square drive.
If the engine is installed, they are sometimes hard to access, because of their proximity to the engine mounts.

Draining the rad still leaves a lot of coolant in the block.
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Exactly, and pulling the coolant plugs in the block rarely get you anywhere. There is usually a pile of foundry sand and sludge inside that is sometimes tough to get out. And watch out poking screwdrivers and the like in there. You could do some damage.