I have a 1958 dodge 400 .. replaced plugs wires point condenser , rebuilt carb etc, set timing .. ran great at Idle but faltered bad at im guessing about 100 rpm plus . someone told me maybe broken or stuck valve spring . firing on all cylinders .compression on all cyl. right at 120 lbs, took the exhaust manifold off and discovered a large hole and several cracks in it. Im hoping this is the problem . bought a new used manifold . went to install it, but gasket set doesn't fit . Fel pro Part # MS 8009 B all the local parts store say this is the right part but its not . Anyone know of correct part # ?? Also Im not sure which motor this is . 251 ?? thanks in advance for any help