What 4 spdragtop said, it's a heat shield to keep oil from sludging or baking to bottom of manifold. One question I failed to as, were any of the bolts noticeably loose on the right side of the engine? Usually on the ones with head gasket issues, the short head bolts on the exhaust side are the ones that are loosest... I am glad you are planning on going ahead and doing it right, there are too few 340 blocks out there as it is.
