I seem to recall having read about someone using an Alfa 156 ARB on his live axle with good results (but not on a G27) ... I think I've read that on the locostbuilders forum.
GKD (www.gkdsportscars.com) can supply an IRS upgrade pack for the live axle G27. Fundamentally the chassis for the IRS is the same as the live axle, you'll just need to weld on the 4 wisbone pick-up points and bolt on the diff mount bracket.