340 intake on a 318?

Makes me wonder why Chrysler decided to foot the bill to make 318 4 bbl intakes when they already had the 360 4 bbl intakes on the shelf. Same thing with the 2bbl's. There had to be something different that warranted casting a new manifold. Later on of course, they used the same manifold, but, the ports between the two heads were a lot closer in size.