I've heard of welding cast iron before. It is very dificult, and has to be done with specific welders. I think it was called a powder welder? I don't know weilding very well, I just happed to have to fix a civic manifold once lol. It would be prone to cracking if not done exactly right. The best way to go is build a header out of stainless.
Did you notice that the exhaust manifold is smaller just behind the turbo for the #5 cylinder! I don't know for sure what that does yet, but I think it has to do with direct exposier to either the turbo or the cylinder itself from exhaust pulses. You wouldn't need that any more if you moved the turbo... Like I said it's probably more effiecaint to go with custom headers.
I would not bother with trying to modify the manifold. Port match the ports, open up the outlet, and open up the part of the manifold on the left side of the turbo. The back pressure on my setup never reached the cross over point @ 23psi of boost @ 7K rpms. I ran 500hp with that stock manifold, how much power do you want to build? Spool was @ 3K flat to 23psi of boost, not to bad in my opinion.
One thing I would highly recommend is getting that Felpro gasket off of your car before you fully assemble the engine. That gasket is not worth the powder to blow it away. The stock Nissan gasket is the best part for the job, and they are not to much to buy. I had very bad luck with the Felpro intake/exhaust gasket, and know of a few others that had bad luck with them as well.
The pic is not my car but I have the same manifold. I also have an external wastegate that I want to plumb into the manifold. I was thinking that it would be great to move the turbo left and install the WG in the turbos present position.
I wonder why nissan put the turbo so far back? Must be a reason? I can't see any obvious space/interference issues?
Cast can be welded with an arc welder, or oxyacetaline and the right flux. Search the net for a product called Henrob 2000, its pretty nice you can weld about anything aluminum, stainless, sheet metal, and cast up to like 1/2" thick. Its all done with different tips on a special torch.
CustomLock said he had no problem weldin gon his exhaust maniforld and doesn't believe it is cast iron - I'm not sure I'm with him on that.
Nevertheless, this wouldn't work on a ZXT because the teering rack would be in the way according to the site the picture was from - although I wish it would be...I have and '83 ZXT and a '75 project car getting a turboed V8 eventually.
For some reason the jap cast iron is VERY easy to weld. I've welded up a bunch of different cast iron NA jap exhaust manifolds with just a regular mig welder and have had no failures. American or german cast iron is a different story...