The piece that attaches the manifold is a spring-loaded butterly valve called a heat riser. It's designed to keep the manifold warm in cold weather driving. This part was doomed to fail, and mine was rusted shut and only allowing maybe half of the exhaust to escape. So I did what any red blooded American would do and cut off all the crap.
I began to clean everything up with a wire wheel when I said heck with it, and just set up an electrolytic rust removal vat. I let it stew for about 8 hours. I could have gone longer but I was impatient.
All nice and clean
5 coats of high temp engine paint
I have the manifold and exhaust flange gaskets. I'm going to replace the studs and nuts with some nice grade 8 stainless. I'm trying to sell the Desoto, but I still am trying to repair it best of my abilities.