[flatheadmurre]

If the coolant doesn´t drain there must be a serious clogging in the pump or something...
At this point some kind of "block bandaid" aproach comes to mind...not much to loose if you have a crack..
Whatever you descide don´t run the engine with water in the oil for any extended period of time.
I would pull the pump where coolant don´t drain and check what´s going on.
Then caustic cleaner in the block to get rid of oil...a lot of flushing...evaporust for whatever rust is left...a lot of flushing...block sealer !
Done right this won´t hurt your internal engine parts and may save the block...if not it´s mostly your work involved and not to much money.
Fingers crossed you can save the block...