Next thing is to pull the thermostat & stick it in a bot of water on the stove, bring it up to the boil & make sure it opens a lot, not just cracks open a little.

While the thermostat is out, make sure the brass pipe that stick up vertically out of the head hasn't been lost & that the spring loaded valve on the thermostat closes it. It is the cold bypass that unloads the water pump when the water is cold.

Third is to stick your finger in to the distribution brass pipe that runs from front to rear of the head & make sure it isn't floating around from front to rear. It's OK if it rattles from side to side a little.

Forth is to pull the radiator hoses & have a good look inside, because it's laminated rubber/cotton weaver/rubber, you can have a flap of rubber blocking water flow, sometimes it will show because it may suck in.

Decent Thermostat function: With a quality thermostat, when you start the engine, hold your hand on the top hose & when operating temp is reached, you should notice the hose go from cold to just bearably hot. My cars take approx. one to two miles for the temp to go from stone cold to operating temperature on a 25 degree celsius day.