Front 9.5J+18 and rear 9.5J +14 just means the rear juts out just slightly from the front (4mm to be exact). For a GTS rear wheel drive it would be better if the rear J (wheel diameter) is thicker than the front. Eg. front 8.5J rear 9.5j. But that depends on individual choice. Depending on your bodykit, fenders and camber setup, Skylines are able to fit a 9J +22. So 9.5J +18 would only mean its 10mm wider. But yeah... do a widebody conversion ;P