Thermostat housing leaking

I used a Mopar Performance chrome one with no gasket, only a thin bead of blue Permatex. I ordered a chrome one from local parts store before the Mopar one, although they look almost identical they are not. The Mopar one sits flat on a smooth surface while the whatever brand one did not sit perfectly flat and would rock from side to side if wiggled by hand. I should add that I use Evans coolant and a 7lb cap.