Very true, water will come out the deck area and run into the oil pan.
You could do it if you are careful, but it will easier with head on. Leave the thermostat out while flushing the block. If you can block the radiator hose inlet and outlets, and one of the heater hose outlets, you can use the last opening to pressure up the block with a garden hose. Then open or remove the petcock and flush out what you can. The petcock is the lowest and farthest back hole in the block, if it runs clean there, chances the block is pretty clean. With the outlets open, one of those solid brass hose nozzles with the pinpoint spray will work good to back flush the block through the petcock opening. Power washers also work really good.
Be sure to drill the 1/8" hole in the thermostat to let air out while filling the engine.
Joe
