Stock 312, Holley 4150 4V 600cfm, ECZ-B manifold, late model OEM exhaust manifolds.  Two questions,  1 inch or 1/2 spacer, any thoughts?  Shouldn't I use a 4 bore spacer as opposed to an elongated two bore or an open plenum? or does it matter? the ECZ-B is an unmodified 4 bore, 1.5 inch, Moroso makes a 1.467 inch, 4 bore, spacer which likely matches the Holley exit bores in either 1/2 or 1 inch fiber or aluminum, not sure it's a big deal but putting it back together and might as well do it right.  Thanks

Ted, Redding, CA