It is most likely an older factory 4 barrel intake. With the cost of aluminum intakes these days I would look into an edelbrock Torquer II intake, as you'll get more out of that than the carb spacer is going to give you.
Post the casting number off the manifold. Holley square bore manifolds are rare and appeared on Corvettes only for the most part (though the full size also used them). A Corvette owner wanting to remove his Torker II for sake of originality would pay through the nose for that manifold if it is a square bore flange and not an old Rochester 4GC manifold with an adapter plate on top of it.