exhaust manifold gasket ???

I haven't actually seen that. The 340 castings were incredibly stable, maybe 'cause they were bolted up right. They certainly were not premo examples having been cracked and rewelded by the D/S outlet and the flapper on the p/s removed - holes bolted and later welded. Yes all welding was done with a long sit in an oven but it wasn't bolted to a spare head during those interventions.