That would one hell of a mounting bracket with a hole that size. Marine/commercial/industrial wouldn't have had the C.A.P. requirement that year, so, why would they use that manifold and not the non C.A.P. version? This casting was mainly a Ca. requirement. All the marine/commercial/industrial stuff that I've seen were only 2bbl engines and I haven't seen any of those with that casting add-on.....as Cuda Al's 2bbl C.A.P. photo shows.
