Vortec heads are a newer style than that of Chevrolet. They areidentified by the center hold-down bolts in the valve cover and thevertical location of the intake manifold bolts. Old-style heads havefour bolts around the perimeter of the cover. There are good and betterVortec heads. A better one is the 906 casting number found under theintake port runners. These, along with others, have a 64cc combustionchamber. The intake bolt holes can be redrilled in the Vortec heads.This will allow the use of the common Chevy cast manifold whennecessary.