Damo, what are you doing mate? Bad advice is bad.
What tyre profile would you run on an R34 GT-T with a 245 section width? 245/35 is too short and 245/40 too tall. If you really want something fat on the back run 225/40R18 on the front and 255/35R18 on the rear, otherwise a good quality 225/40R18 will be up to the job on a Skyline without ridiculous power.
OP also stated that he is looking at getting decent tyres, if he opts for a Toyo Proxes R1R (which I can't reccomend enough), Yoki Advan AD08 or similar these will have a wide contact patch for their respective sizes i.e a 255/35R18 R1R is usually as wide as a mid-range 265/35R18.