Had the sane problem with mine, leaking out the head/manifold gasket at the crossover. The gasket I used was almost like a black thick pressed felt. It adsorbed the coolant and wicked it out to the bottom of the manifold. I had a film of sealant around the port in the head and manifold but the coolant traveled through the middle of the gasket. Gasket worked well sealing the intake ports but coolant soaked right into the gasket and leaked. SCE intake gasket 128103, forget what I replaced it with but no leak now. Coolant also wicked to the intake bolt holes in the heads and corroded those.

Those aftermarket chrome thermostat housings always seem to leak. An aluminium one can be polished to a chrome like finish, if one wants it shinny, and it wont leak like the chrome ones. My brothers "69 Roadrunner had a Chinese made Holley housing that looked like it was part of a locomotive boiler. New gasket and a tube of sealant wasn't going to stop that from leaking, putting it on a belt sander to take the curvature off the mating surface helped.