Chevy 350 Thermostat change & other Tips
A simple tip I do when changing a thermostat in an engine, is to drill a hole, less than 1/8″ in the thermostat housing. This way when you drain the coolant/anti-freeze by detaching the bottom radiator hose, all the coolant in the engine block will drain too, because the tiny hole in the thermostat is enough to allow air to come in from the top. Additionally, when you fill the cooling system from dry, that small hole in the thermostat allows you to fill the cooling system completely without having a problem with air pockets in the cooling system. A little extra attention to detail makes things easier in the future.
The fastest way to check thermostat is to put it in hot water and see if it opens up. It is best to check the thermostat before replacing it, even if it is brand new.