How to drain lower water jackets on Dauntless

After flushing the coolant system and dumping the flush water out of the petcock on the radiator on my v6 I relized the bottom radiator hose is higher than the water jackets that the freeze plugs are in. On the passenger side there is a small plug with a 7/16 bolt head on it next to the freeze plug and I assumed that it would drain the rest of the block, but the plug did not budge and I did not want to take a chance on twisting it off. Am I correct on this plug draining the rest of the block or is there another way?

 

I did and twisted the 3/8 to 1/4 reducer into, afraid to use any thing larger since it is not that critical,Thanks for the reply and confirm my assumptions.