Jlathem, from my experience, what you have found is exactly the normal situation: That is, GM engine plants in general do not use any gaskets on the exhaust manifolds. (at least on their V-type engines) And I have also found that almost all (if not all) aftermarket gasket sets include gaskets for the exhaust manifolds.
Also, on the ceramic coatings, I have used on steet tube headers, I really didn't notice hardly any decrease in the external temperatures. If it were me, and I decided to do the ceramic coating, I would still darn sure re-use the heat shields. If you don't, you will be asking for problems related to radiated heat from the manifolds. -By the way, you could have your heat shields coated too- -they would look better,,,
And the guys are exactly right about the heat tape. (header wrap) It will greatly reduce the extermal surface temps. Just be prepared for accelerated deterioration of the headers/manifolds themselves over time, due to much higher internal operating temperatures of the manifolds. And, depending how tough the parts are, you may never experience the deterioration.
Good Luck !!