How do I seal the Choke Thermostat?

That manifold/choke setup was only used about two years. There has been some discussion about it over a slantsix.org. Go there and use the search function. If I remember correctly, the metal cup, is no longer available. If you find that, the gasket is not a big problem. One solution, is to change the exhaust manifold to one that doesn't have to opening.