I saw a old video somebody posted a link to on here awhile ago and it told what the exhaust crossovers do, it said the crossover not only did warm up the plenum below the carb but actually kept it hot, the large dropplets of fuel (that we don't want) where suppose to drop down on the hot plenum (hot from the crossover) and they would vaporize by the hot surface and be sucked into the cylinder and then burn, probably needed more in yesteryear when carbs where everyday used. Anyway blocking the things off is usually done, edelbrock made cast aluminum plugs for some motors and I have heard of jamming aluminum foil in there packing it tight (Not my idea and I haven't done it) keep in mind thats exhaust and it gets really hot, I even thought of and heard of using some sort of refactory to help keep'em from leaking.