I would think the rules call for a 2 bbl carb, does it ask for a stock gm carb ? you can buy a adaptor platethat can be used to adapt a 4 bbl to a 2 bbl manifold and vice versa. then you dont have to deal with changing the manifold
some rules tate only that it has to be a 2 bbl manifold and then a big old holley 2 bbl is a good way to go