G'day David
Interestingly, your GS (178-433) sits sandwiched between two SASR LRPVs (178-432 & 178-434) in the ARN list. It is definately not an LRPV but may be a SASR LRPV support vehicle. I say this as the extra tank modifications are the type of mod the SASR would undertake. It may be a long bow to draw but who knows
.
Before the Unimog 'Mothership', the SASR utilized a number of GS Land Rovers as support vehicles to carry fuel, spares and supplies for the LRPVs and their crews. These support vehicles accompanied the LRPVs whilst on exercise so would require similar long range fuel capacity. Additional to this, the under tub long range fuel tanks, as installed in your GS, would allow for maximum storage in the tub.
Inspect your Land Rover carefully for 'oddball' holes etc as these may give some clues to it's past history.
The photo below shows four of these support Land Rovers whilst on exercise.