'70-'72 Slant-6 exhaust manifolds have an open hole at the top of the choke tower on the № 5 runner. A stainless steel cup fits into this hole, and the choke thermostat goes on top of this cup. There is a gasket between the cup and the exhaust manifold. The same arrangement was used on '70-'72 V8 engines, but with a differently-shaped gasket. The Slant-6 choke gasket is not available. You can scrounge around for a new old stock item, or try and cut one out of high-temp gasket material.