If you decide to block it off get the valley pan that blocks it. You can epoxy a thin sheet of aluminum to the manifold over the crossover ports (my father did this back in the day on a 340) but the valley pan would be best. I would also look into getting gaskets designed for blocking the crossover. I don't know if they are available for a big block because I am a small block guy.

Ryan "Bigs" '73 Duster (BLKDUST) - Black, 100% factory sheet metal, flat hood, 346 cid, J Heads, and a bench seat. http://s268.photobucket.com/albums/jj1/bigsbigelow/ Best to date: 12.40 @ 109 mph