Howdy KR:
And welcome to The Forum. Jack is right on , as always. but here are some more details. While the exhaust manifold is off check the mating surface on the head for warpage. You probably won't find any and without removing the head to have the surface milled flat, you can't do much about it. The exhaust manifold is a different deal. You didn't say what year casting you are working with or what condition it is in so. So, whatever it is check it closely for cracks and or broken ears. If it is solid check it for flatness on the mating surface. If it is within .010" + or -, it is good to go. Personally, I'd have it milled flat just because that will afford the best chance of a good seal. Reinstall with a quality exhaust gasket and properly tighten the bolts as outlined in the service manual. After it has been through several heat cycles recheck the bolts for proper torque. check them again in a month or so.
I hope that is helpful to you. Let us know how it works out.
Adios, David