But surely thats alot of hard work fitting the 2wd rear beam just to still use the smaller 4x4 7" diff!

all that swapping and changing of the beam and output flanges etc, and then still using the 4x4 diff which is now mounted with spacers! the only real benefit after all that work is stronger flanges and driveshafts, when all it see's is 60% of the power anyway!