It would be best to consider getting/installing an OEM gasket Merc part number 27-530451 or the temp "readings" may fluctuate considerably.that might be a problem, my thermostat housing does not have the rivets. There is signs that it had them but no rivets in place now. and my temp sensor is in the housing
Most have the temp sensor and switch in the housing unless it is really old.The only time you need to use the gasket with the grounding rivets is if you're water temp senser is located in the thermostat housing.If it's located in the intake manifold it's not so crucial..