Though it is not really a drain, you can remove the temp sending unit so that you can flush out the block. It is located near the thermostat between two hoses. It is on the driver's side of the engine, above the V and below as well as a bit foreward of the throttle body. It is much easier to remove that the thermostat. With the sender out, you can add water to the water bottle until there is clear water coming out of the sending unit hole with no more coolant color. You should still drain the radiator from the petcock, probably both before and after flushing through the sensor hole. Five quarts of coolant and the rest water should put you close to 50/50 mixture.
