Would be no harder than converting a live axle VP, VR, VS to IRS ie. weld in the appropriate (sheet metal) x-member support brackets and bolt up the rest. Shorter tailshaft.
But the track width will be wider using all VR/VS parts. Using VP/VQ IRS bits and standard offset rims will reduce the track some but will change (narrow) the track (of the new "axle") by 34mm which is more than the 25mm allowed in NSW and Vic. (not sure of "federalised rules" or if they apply yet). It will also screw with the suspension geometry. A shorter virtual swing arm length is one effect.
The following may help:
VL sedan: Front track- 1451mm, Rear track- 1433mm, wheel offset- 43mm+ve
VP/VQ IRS:FT-1485 (=1451+34, VL and VN/VP front suspension are very similar), RT-1487, wheel offset- 26mm+ve
VR/VS IRS:FT- 1491, RT- 1491, wheel offset- 43mm+ve
VP/VQ IRS with most VB-VR/VS wheels:RT- 1453