With iron heads and manifolds I don't use a gaskets but use a smear of high temp silicone. The surfaces must be clean and relatively flat for it to work. Also give the goo 24 hours to set.
Headers - use Fel Pro race series.
The factory machined surfaces were extremely flat and everything torqued correctly. However, if the cylinder head exhaust surface and exhaust manifolds aren't too flat anymore, Remflex exhaust gaskets hold up pretty well.
If you *need* gaskets because of leakage, warp, age, whatever, then take your pick from several popular brands. Don't be surprised if they blow out on you. This is why many prefer to make the OEM no-gasket scenario work, if they can.