That's a great deal. The kind of deal I look for...lol. There are 3 generations of the poly 4bbl manifold. 56-57 used a WCFB which was a smaller cfm carb, (about the flow of a large 2bbl). 58-60 manifolds had a larger mount for use with an AFB carb and a generator mount. 61 and 62, (last year for a poly 4bbl), did not have a gen mount and are the hardest to locate. All this is for the purest restoration folks. (most forward look restorations lean this way).
From a performance out look the aftermarket aluminum manifold is the best choice. Around the $500.00 mark. Heck... the OEM manifold alone weighs 50 lbs.
Yes. Dual 4bbl manifolds from the era are much easier to come up with than the single 4bbl units. The poly 318 Hi output, (4bbl. special cam and dual exhaust), was always a low volume option. Most just stepped up to a larger engine.
I'm sure this fellow got his setup up for a song and dance, but he knows the market.