The later 235s used cast in the block galleys to deliver oil to the rockers. There is one drilled hole that needs a plug, and that's on the outside of the motor. There might be a crack anywhere... but from reading the OPs description I'm lead to believe that the amount of coolant being lost is significant and immediate.
Since its not coming out the exhaust, that leaves a crack in the water jacket or a head gasket that is not sealing properly.
I can't think of anything that penetrates into the block water jacket from the inside.
