Another Thermostat / Housing question.

You want it to sit flat inside the lip.... perfectly... it suppose to fit tight in the lip. Is it not seating flat in the lip or just moving side to side? If you can get it to fit inside the lip, glue it in, place gasket on manifold, and place thermostat housing with thermo on top of gasket and then tighten it down.
Everything you have will work.

Take a pic of the thermostat sitting in the thermo housing.