Exhaust gasket replacement failure.

www.pentastarparts.com.au has the gasket you need to seal an old manifold. It is a composite gasket that is graphite coated and will seal most anything. They also have an intake/exhaust to head gasket made from the same material. Search the website for hotbox gasket and/or intake gasket. They can have one to you in around a week even though they are in Australia. There is a box to check on the website to get prices in US dollars.
The broken stud is fairly easy to remove. The stud is 5/16" so I just drill a 1/4" hole and retap for 5/16-20. The end holes will be into the water jacket so remember to coat the threads on the new stud with some sealant or you will have a water leak. Bolts and nuts on the exhaust flange is not that uncommon. My '66 Valiant is like that and the manifold I pulled off a '70 truck is the same way. Put them together with hardened lock washers and you will be fine. Hope this helps.