Dale:It appears to be a combination of corrosion from exhaust and moisture, plus burn through from the exhaust heat. Is that where the choke heat tube fitting was? If so, the heat tube(s) are probably also rusted out. The problem is that there may be more weak spots. A patch can be welded in, I've done it. Also, you could block the heat passage at the manifold/head juncture and no exhaust would get to that area. If you do that, block the heat riser valve open. If you drive in cold and wet climate, you really need that heat to prevent carburetor icing.