Yes, you need to use a pipe plug Msocket. The drain plug on the side of my 88 XJ block. You need to buy the 5/16" pipe male plug socket.
Buy the socket from who ever you want but to help you out here a picture :
http://www.mytoolstore.com/sk/sk03221.html
I tried to grind down something to fit but it did not work for me, I tried the pipe plug socket a buddy of mine picked up off a tool truck for me and it goes into the square hole and I can tell it is the right tool. Next time I change coolant I'll pull the drain plug using the pipe plug socket.
If you can't find the tool you can fill the engine with water and drain it. I figured out if I was getting 2 gallons out each time I'd have to fill and drain the engine 4 times to get the concentration of old coolant below 2%.
I would pour in 2 gallons of distilled, let the engine idle for 5 mins, then let it drain. It took me an hour or two to do the 4 fills and drains of distilled.
I used distilled water because a coworker who use to work at a Military Nuclear reactor told me to only use distilled. Distilled keeps you from introducing chlorine, plus all minerals in your local tap water, these can deposit on your block and radiator. The same guy told me how this one nuclear plant on the east coast had to shut down and replace the pipes because the utility did not monitor chlorine content.
At 89 cents/gallon of distilled the peace of mine was worth the 5-10 bucks it cost me to buy the water.
If you need to get the job done today consider the multiflush with distilled.