Give HiTech Motorsport in Drury,Auckland a call (09)294-6001 as they make a cast manifold for twin side drafts for a 20V, probably have them in stock as well, otherwise it'd be as above you'd have to fabricate one.
Pretty sure the redline ones are all 16V but in saying that I haven't checked if they do a 20V one yet as I already know where to get one from at the right price if I ever was to build a 20V with carbs(Have seen one in an offroader here in NZ)