I have been fortunate that my manifolds and heads are pretty flat and smooth. You can dress them with a sanding disc. Or use a schmear of high temp silicone to help seal. Even the 1971-72 manifolds/heads that are missing the end bolt holes can be used if you bevel the surface on an angle to press tightly against each other. Worst case you can use soft header gaskets. Normally the originals are just rusted away as the OP mentioned. So far the steel ones have worked well for me.