The original (1956, 57, 58) factory Poly 2 X 4 intakes were cast iron and used Carter WCFB carbs. They're often on ebay for $300 - $600, often easier to find than a used single 4bbl intake for those engines. Note: the 1956 was only a 277 cid version.
If it seems like dual quads are overkill on a 318, they likely are but those carbs (2631S (front) and 2632SA (rear) only flowed about 380 cfm each. Remember, back then even a little 265 Chevy smallblock in a Corvette could have factory dual quads so it wasn't that unusual of an option before larger engines were developed. The Poly engines with this intake also had dual exhaust, higher compression and better cam. I don't know if the heads were different or not.