Hi Adam. An R31 rack will bolt into the crossmember,but I'm not 100% sure it will fit the splines on the steering shaft (would be very unlikely to be different) As you probably know,the HR31 doesn't use a rear rack,so you would have to lock the rear cradle up,either by fitting solid bushes,or replacing the rams with a solid bar or such like.
Oh,and good luck trying to remove a 4WS rack while the engine's still in there!