Hey mate,
I've got a GTS-4 which I put 30mm wider front guards and gtr rear quarters, I'm running 18x10 +18 on front and rears, the front sit nice and flush with the guards. But if your going 50mm wider at rear you will be able to run something like 18x12.
This is what my car looks like and for those following it all you see is spinning dish, but please forward pics of your car man I was going to build my car based on that same picture you've got there I still want to do a Super-Gt style kit for my car down the track.
If your interested these wheels are Work Emotion XD9's, go to workwheelsaustralia and check out what they have if any are 18x12 to fill your guards.