The rear end conversion "kit" from Monarch is £1250 plus you need the E30 rear end in the first place, plus new alloys at £300 each. It sounds a lot, but I suppose you get a diff (reputedly) capable of taking the torque, new half shafts (Stag ones are a bit crap and suffer from spline lock up) and rear disk brakes, all of which would need to be done individually otherwise.