This manifold is the same as P/N 12363406 except it has been "CNC" port matched for GM aluminum oval port heads used on all big-block Chevrolet engines with large oval port heads (1975 and prior), and all aluminum heads with oval ports. This manifold includes the latest design in dual-plane manifolds and has a Holley carburetor bolt pattern. Use intake manifold bolt kit P/N 12367959 which includes 16 bolts (8740 chrome moly 3/8" - 16" x 1.5" with 3/8" hex head and 16 washers 5/8" O.D.), and manifold gasket kit P/N 12366985. Technical Note: Manifold has a bolt pattern for air conditioner brackets, late model water neck, and alternators. It will not fit production Corvettes, and it is necessary to check hood clearance on Chevelles. (Manifold height is 4.45".) This part is the same as P/N 12363421, except it is designed for use with the Holley carburetor.