Ah, OK. I don't know that there is any reason for a bypass with a carbed system. I think they are for certain types of EFI as to not confuse the computer. I dunno about the radiator...it might work. I've never heard of anyone using them. They'll hold up to 15 psi (at least I've seen radiator caps that high).
I'd like to restate that an air filter enclosure will not work. Look at what Linc and kirkallen have done, that is what you need. The one that kirkallen used still blew apart when the engine backfired.
This will give you an idea of what I'm talking about:
This particular one won't work on our engines but the design is there.
Here's Linc's 100% homemade bonnet:
Notice they are beefy, sealed designs.
Anyways, what year GN is the turbo from? It is different depending on pre- or post-1984. Before '84 the system was carbed and draw-through, and after '84 it was a blow-through EFI. Both had electronically-controlled wastegates, though.