400 Exhaust Header Gasket?

Finally getting around to the mountain of work my late Dads 79 needs and I can’t for the life of me match up the shape of the header gasket to anything I’m seeing online. There’s an exhaust leak developing that I hope I can cure with a new gasket.

I know he had a nice dual exhaust made up for it but I don’t know much about these headers.

Can any of you experts tell which gasket I need by looking at these glamour shots of the rusty headers?