At this point, you need to do a lot more research on your own with some professional engine builders if you're really serious. You are looking a a ridiculous amount of time and money to produce that kind of power from a Nord. A race-prepped Alfa Nord engine putting out 180 - 200 reliable horsepower is probably going to cost $7,000 to $10,000. Just reworking the head on a race engine will run $2500 or so. What you are talking about in building that kind of engine is going to cost a lot more than a typical race engine and will probably never be reliable enough to drive regularly.
The basic Alfa Nord engine was designed more than 50 years ago. It started at 1300 cc and was ultimately expanded through 2000 cc without a major redesign. Even with with the Twin-Spark head, it never went above 200 or so HP. Once you start going ant serious number above that you need to start looking at serious modifications to the block to keep the cylinder sleeves from moving around and keep the head bolted on. You'll need special headwork and special headgaskets, as well as special sleeve and special pistons, not to mention the turbo and injection systems.
Bonneville 2004 - Two Records
Note that there is no real "off-the-shelf kit" for putting a turbo on an old Nord engine. You will need to find someone who has done the development work and pay them, or spend some years doing your own.
Then there's the fact that Alfa transmissions from 105/115's won't handle that kind of power, the cooling system will have to be modified, and so will the brakes, the diff, and the suspension and probably the chassis.
The bottom line is that, yes, 350 or 400 hp is possible but unless you have some particular need to do it with an Alfa Nord engine, or most 2 liter engines for that matter (Jim Steck had such reasons) , you're crazy.
You can probably build a nice Chevy small block with 5 or 6 liters of displacement and pull 400 hp out of it for $3500 or less, and still drive it every day. If you need that kind of power, that's a much smarter plan. Alfa 105/115 Spiders were never muscle cars, even when they were new.
I posted the link to the Bonneville Alfa to give you some idea of how seriously you would have to take the project to achieve 400 hp. I suspect some wistful daydreaming on your part, and while I don't wish to discourage you, I don't want to lead you on either.
I leave the rest of this discussion to others.