If nissan were to put a V8 into a GTr, they would have only 2 choices.
Either use an infinity Q45 4.5L V8 or use a de-tuned R390 LM engine 3.9V8-TT, which uses a magnesium block.
Keep in mind that Nissan are still trying to get out of the hole they got themselves into in the 90's, they cannot afford to devote one special engine for one car alone, but modify an existing engine. A V6TT is the best choice, in my opinion.
Notice how they like to use the their new V6 from the 350z. they use that in the G35 sedans and coupes at the moment, and also in 4wds and anything else they can cram it into.
That current pic floating around in one of the current issue aussie magazines, is a picture from the internet, and is a photochop, ive had that pic on my HDD from months. Remember, they WILL change the design of the car all the way up to the last minute.
The 350z was change in the final months before its release. Its relase date (in the US) was known before the final design. Though those final changes were small, there were still changes made.