Why do you yanks like going backwards in technology? Z's have twin SU's(1.75") from factory... now a set of bigger(2") SU's with ram tubes would be a far greater paerformance upgrade and cost less then going back to down drought technology!

Ive seen this setup a few times on classiczcars.com... i cant understand why?

You are talking about fabricating a new manifold? If not, where do you buy such a setup for the Z-car and is it really less expensive that $750 USD? Brand new this kit is only $875 USD -- and it includes everything, every little part, no fabrication needed.

There's an even better picture of the kit here:
www.redlineweber.com "America's premier distributor of Genuine WEBER Carburetors for over 30 years".

Remember, each of these carburetors flows 330 CFM. What do 2" SUs flow?

I read the specifications for this kit and the reason Weber developed it. 2 downdraft Webers are better than 2 S U's.
This set up could use two syncronious 38 DGS instead of two 32/36 progressive DGV.
About the Z kit, I can see the posibility of using a kit like this were a SU manifold is present. Other than that it would be cutting a stock manifold and going from there. The application that I see here is in case of a SU manifold around.

About the old technology comment, in my part I am not bent on old technology that does not work, but on fun and never ever done projects. I know I could get a 20B 3 rotor motor and put it in my car....but I seek fun and knowledge besides looks or a show off motor. I appreciate simple things like investigating and trying to implement a dual downdraft set up that I have never seen done, for a street car, my car is no racing car.
The IDA Weber, the DCNF ( the F corresponds to Ferrari, used in sets of 6 for the 12V Ferraris) are all DownDraft Webers. There is nothing obsolete in downdraft carburetion.
But again, I am not trying to find the ultimate performance set up, but a fast, reliable and different option.In case of LHD datsuns, many times a dual DCOE is simply unfeasable.

At another forum some dude told me there... ( by replying to my posting )... is a company in Europe that made ( I hope to get the link , hopefully it "makes") adaptors to use DCNF Webers on DCOE manifolds.
Aparently that would be a short DCOE manifold on the engine and the adaptor bolted on to it to receive the DCNF.....