Use a shop vac to suck the coolant out with the thermostat removed but both pipes and the water pump installed. That way you can create a small vacuum within the engine. I use this technique all the time when rebuilding snowmobile engines and have done it a couple times on the z. It sucks 90% out of the engine. Only other way to get more out would be to pull the motor and turn it upside down. I would avoid pulling frost plugs with engine installed. They are very easy to screw up and pound in crooked or not be able to pound them in with enough force to get them in. If you do use the frost plug technique do not use silicone to install them. Silicone can be a lubricant once wet and the plugs could pop out. Use aviation gasket maker. It never really dries and is super sticky, works awesome.
