When I removed the thermostat housing from my car, I found it somewhat difficult to break free. If this happens, once the nuts are loose you can strike it with a rubber hammer (or other suitable object). This got it off easily for me.
The most annoying part was getting the coolant into a bucket neatly--hard to do without a radiator drain plug. (I let it leak out the top of the loosened hose.)
Anyway, should be easy to do.