Freezing temperatures - water in engine

I have a 68 dart 340 small block. Winter has snuck up on me, tonight we're supposed to get 20' temperatures. We got the motor running with just water in an after market radiator. I have a thermostat in. I think I'm gonna have to pull the water pump so I really don't want to put antifreeze in. I've drained the water from the radiator, turned the motor over.

Do I have to worry about freezing in the block, with just draining the water?
Should I put some antifreeze in? I am guessing blocks crack because the system is under pressure. Thanks in advance.