The manifolds can all be immersed in parts cleaning solvent. Carbon buildup in the exhaust manifolds can be removed with carb cleaner, wire brushing, or glass beading.
Check the manifold flange and gasket surfaces with a straightedge. Replace any manifold with warped, distorted surfaces. Do not machine either type manifold in an attempt to salvage it.
Minor scratches and nicks on gasket surfaces can be smoothed with 80 grit emery cloth, an oil stone, or fine tooth file. Inspect the manifolds for cracks on all surfaces. Check the exhaust manifolds and exhaust inlet pipes for splitting and flange or joint separation. Although seriously damaged parts should be replaced, minor damage on low mileage steel exhaust parts can be repaired by welding.
Inspect manifold fittings and threads. Replace loose damaged fittings and repair thread damage with a tap or helicoil insert.