You are correct, it's marine. Other than being on the heavy side it will work fine on your 318. You might have to adjust the floats in the the carb though because it will sit tilted forward a little.
It is also a 1965 intake which has smaller bolt holes and at a different angle than 66 and newer intakes. Don't use this intake. There are many other intakes that will work better without modification for you. Rob has a good suggestion.
