Here's a quick test to perform , start the engine cold , hold squeeze the top rad hose or thermostat hose and let run a few mins , you should feel pressure occurring and if not the thermostat is faulty.
I'd check the specs of the thermostat needed and buy some high temperature gasket maker too.
Remove the top rad hose bolts at the thermostat housing and have a container handy to catch the coolant, then pull the thermostat out , place a soft dry cloth is it's place , remove the hose from the housing , dry and clean the gasket surfaces and the housing with a paint scraper and fine sand paper, clean the inside of the hose and dry it,wipe away any particles from the thermostat area, remove the cloth, add new thermostat right way up, add a beed of the gasket maker to the centre of the thermostat housing and then attache the housing with the bolts so it's a firm tightness , then use a new hose clamp to attache hose again , then top off the coolant , start engine up , watch for any leaks and tighten up bolts slightly if needed and keep topping it off , be sure you test the anti-freeze with a anti-freeze tester too to make sure it's not mixed more then 50/50 with water.