After I saw some photos of a VX manifold and gasket, it looked like some port matching would help with potentially cracking manifolds. There were apparently, based on the markings from the gasket on the manifold gasket surface area, areas where the cast iron manifold would interfere with exhaust flow, particularly on the inside of the two center exhaust ports. Much higher temperatures in the center of the manifold, combined with the proximity of the center ports to the catalyst COULD be instrumental in the manifold cracking which seems to be almost universal at some point in time.
http://ecomodder.com/forum/showthrea...ory-32194.html
regards
mech