Damo_R34 on SAU managed to fit 18 x 9 +15 front, 18 x 10 +20 rear. 225 40 front tyres, 235 40 rears.
Maybe send him a PM to see if he needed any guard modifications or suspension modifications.
http://www.skylinesaustralia.com/forums/Damo-R34-m47366.html
If you use the Wheel offset calculator
The front wheels will have 3mm less inner clearance, and extend an extra 9mm on the outside
and the rear wheels will have 14mm more inner clearance, and extend an extra 2mm on the outside