i'm of the mindset, that if you build a car with everything IDENTICAL to the factory specifications, you may then then add genuine badges (not shitty ebay ones) and then re-badge it. i think that you should also add your own touch to the badging, for example if i bought an r33 GTS-T and replaced everything in it with GTR components (ie everything bar the chassis and shell) i would then badge with the iconic GT-R badge, but i would probably add a small silver sticker underneath with like "backyard spec" or some shit like that, so if anyone queries it, you can say "not a regular gtr, but a deadly backyard built one or whatever. problem is that ricers with gts NA skylines and NA silvias and all the other garden variety NA imports rape the idea of rebadging, not as a tribute to the original, but by badging it for status (or to pick up wheel biters).
the xy gt questions, as long as you're building it as a tribute to the australian icon, thats cool, just add your own touch, so you don't take away from the thousands of dollars spent by an original owner, just to differentiate your car from theirs. I mean, if i had a gtho phase 3 and someone in a 6cyl had kitted it up to look the same and used the same badging, and i'd forked out 500K or whatever, i'd be inclined to screwdriver them off and give them a mouthful.
but as long as its your tribute, and you spin it your own way, and you can tell the difference between it and the genuine article, i dont see why both couldn't be enjoyed in their own way. even though we are japanese car fans, everyone can appreciate hundreds of hours lovingly spent restoring a car of any kind.