spray paint for slant 6 exhaust manifold

If you want the manifold to truly look original sand blast it and bolt it to the engine before it's painted. The manifold will end up covered with varying amounts of overspray which will burn off and be replaced by a light coat of rust.
If you want to "detail" the manifold, Jet-Hot offers an iron grey ceramic coating which would be costly but permanent. Calyx manifold dressing is economical and works well, but must be reapplied periodically.