Is the Offy the manifold that was/ is run on the 343hp 383 in stock or S/S? I would imagine the iron unit that came on the 62-3 B- bodies was the same intake as the 58-59 low decks.
I cant answer your question but the tech at the time is apparent. The factory dual quads by Chrysler- the Offenhauser units and a manifold made by Weiand all have the same design. A single plane large plenum. The Edelbrock D66 for the 273 and the Edelbrock CH28 for the 440 are familiar dual plane designs. Offenhauser dual quad manifolds were used to make roots blower intake manifolds (and not for just mopars) by many companies. The open plenum design worked well for this and machining/welding/ converting one was cheaper than casting one from scratch.
