Yeah, from what I've seen in my calculations, there's not much you can do. You're gonna have to put on some stretched tyres and put some neg camber or just flare your guards.
I never realised how high of a offset you have to run on skylines. Like you said your current wheels of specs, 18x8 +35 with 230/40 are pretty much flush with the rear, these are 18x9 +15 , 225/40 with neg 2 camber on a 180sx:
With these wheels and tyres on your car, they'll stick out and extra ~35mm.