You will need the following
552174 Fuel Tank
PM267 Fitting Kit
90560612 Sender Unit
543764 Fuel Neck Hose
277259 Filler Tube
NRC6905 Fuel Pick Up Pipe
ESR3277 Fuel Pick Up Pipe Gasket
526783 Two Way Fuel Tap
510267 Sender Switch (2 required)
526786 Fuel Tap and Switch Bracket
DA2019O Right Hand Fuel Tank Outrigger
330366 Fuel Filler Neck cowl
Now you will obviously have to make up a new pipe from the pump to the changeover tap and then a couple of pipes from the original rear tank and the new side tank to the changeover tap , you would also have to do the wiring to the fuel gauge using the new switches as the original items from the optional parts catalogue are difficult to obtain.
The fuel return can stay as it is going back to the main tank in the rear, it makes it simpler than having to split off the return, however it does mean that the main tank will fill while the "reserve" tank empties
Using this method then it keeps it standard Land Rover and is a well tried and tested system used on all military vehicles and the preferred option of the overland expeditions of the 60's and 70's
Rob