There would be no such thing as a Magnum manifold before 1966
and especially one without a divider.
1960-65 401 and 1962-65 478 small port heads engines used a semi hi-rise manifold that used the larger Bendix-Stromberg carburetor, this is actually an excellent upgrade for a 305 V6 if you were to go with the Holley 500 with a trapezoid adapter. If you plan on converting one to a 4bbl then you need the 1966 on up Magnum manifold which has the divider. Included are some pics of converted Magnum manifolds to 4bbl