I have a 67 GT 390 Fastback, As i speak the exhaust manifold is off the engine and to my horror the gasket set (for a gt390) is the wrong fit, i am attaching a picture of the new and incorrect gasket and the old gasket, if anyone can throw some light on the situation it would be a great help.
Really jealous of that car man. Awesome. Can't give much advise on gaskets here from the pics. I would probably try napa or another parts house. You're gonna have to match them up. Might take some work. If that is an original set up you have it's kinda rare so parts houses are gonna have difficulty looking them up on their computers. Take it down there to match it up. I recommended napa because they still use master parts catalogs along with computer catalogs. Napa gold is a great exhaust gasket also. Good luck.