my 400 ( 72 model in my 69) is run w a Holley 650, and runs great...you dont want too small a carb,...on the other hand you dont want too big either, bigger isnt always better...should be 'just right'///

open hood scoops is not doing you any good unless you have RamAir (and barely then...since it actually doesnt do THAT much...)
unless you have RamAir , I`d keep them closed...after you paint them just paint the "open" area black and it looks open,, no need to get crud or water in therejust above the carb...Steel scoops recommended....