I used one of the lines that come off the heater core. You're gonna end up stirring alot of shit up that's gonna end up in the thermostat. I would just dump the mud and run fresh water through it a couple times. The thermostat is a bitch to get to but while it's opened up, bypass the heater hose that runs through the throttle body, they are a known leak spot. The heater core pipe that runs across the front of the engine should be replaced also.
When i took my front cover off all the gaskets were gone. The dexcool just ate right through them. Unfortunately one of the cover bolts probably hit a water jacket and i could never get the cover to stop leaking again.
This is a very informative site for the 3100/3400/3800 motors.
http://60degreev6.com/
Sorry to ramble on but i've never ridden in a car as smooth and quiet as my century. My car did 200k hard miles on NYC streets and highways. I used to hit potholes the size of fiats and never got a flat or got the suspension out of whack, car was a friggin tank.
