I have a 2000 malibu with a severe coolanyt leak coming from the back of the thermostat housing. How much do I have to take off to get the housing off to reseal it with permatex?

Changing the thermostat will probably take you a few hours. The plenum and everything associated with it (EGR, MAP, power pack) needs to come off. I believe you may also have to disconnect the fuel line......it's been a while since I did mine. The problem lies in trying to get to the one back bolt. Get ready to curse.

might aswell change the intake gaskets too