Well, first off, it would be cheaper to build the 454 that a chevelle SS comes with than to do either of those. You can get aftermarket fuel injection, you can get adapters to put a 6 speed gearbox behind it, cruise control, air conditioning, etc. There are aftermarket brakes and suspension setups you can get. You could build a Chevelle that would out-accelerate and out-brake a GT-R for less than the price of a GT-R engine. Navigation and such is easy. There are numerous companies who make in dash nav systems. Pioneer, Blaupunkt, Sony, Alpine, Kenwood...There's tons.
Now, if you're serious about the full tilt swap, first I'd say stick with GM, and ditch the nancy-boy nissan parts. They just wouldn't look right. Second, start with a regular Chevelle. The SS will cost you far more to buy, just to rip apart. And if you stick SS badges on it, unless you enter it into shows where originality counts (which obviously you wouldn't since you're talking about bastardizing it anyways), nobody will know the difference between if you started with a real SS or not unless they can read the VIN.
Bottom line, to do what I would, you could get out the door with a decent paint job for less than $50k. To do it your way, with sourcing the ZR1 engine, real SS car, etc., expect to double that easily.