do yourself a favor, and snag a thermostat housing off an older mk1 car, there solid aluminum. you can drill it and tap threads for 1/8" pipe, and screw a plug in there..
all my cars have this mod, and its a real time saver. take out the plug, and screw in a 1" chunk of tubing and it drains cleanly into your bucket..
now that you have the coolant out, replacing it should be easy..