Been through this a few times over the last 30 years or so...had porcelain on one car, and when they baked the headers, they straightened out a little bit and the fit changed; later on the porcelain developed little cracks like lacquer-checking.
The Jet-Hot kind of stuff seems to last forever, but you need to have the header fit finalized beforehand.
I've used Eastwood's Stainless Steel exhaust paint on a couple of cars' headers - sprayed on, reduced a bit with enamel reducer - and it worked fine in a dry climate. Gotta bead-blast first, but whatever mods you need to make can be touched up by gun or foam brush.
I always cut & trim back the header flanges to eliminate warpage & increase plug access.