This fella has a bunch of parts and a 2+2 in Queensland.
http://www.ebay.com.au/itm/DATSUN-260Z-AIR-FILTER-BOX-HOLDER-/120723629406?pt=AU_Car_Parts_Accessories&hash=item1c1bb0655e
A smarter bloke might have made a sand cast of the middle manifold and made one to suit SU and Weber pattern. Make it to a 4 cyl Manifold spec and then you can turn a dual to a triple.