It might depend if the water pump housing or the thermostat housing are machined for the thermostat. If the water pump housing is machined, the thermostat goes in then the gasket then tstat housing. Just the opposite if the t stat housing is machined. It looks like your T stat housing is steel and not cast and that the thermostat goes in the housing and the gasket on top. I use gasket sealer to keep the gasket in place before bolting it down. Check it out before bolting it down. Figure out how it's supposed to work and pay attention how it comes apart.
