1966 Ford Mustang leaks from intake manifold

1966 Ford Mustang mileage: 221,000.I was replacing the thermostat and put it back together with the housing and intake cleaned free of left over gasket with a air-powered tool and material to clean metal, put on the new gasket with The Right Stuff sealer and new thermostat, started to fill with water and it started leaking out of the passenger side of the housing. I took the housing off again, cleaned everything again, got a new gasket put everything back on and it leaked again. Took everything off again, cleaned it again, tried to make sure the metal was smooth on the housing and the intake, put it back together with the second gasket and more sealer, waited a few hours then filled with water and it leaked again from the same side. What to do now?