What you have is a universal sending unit intended to be fitted to a large number of different cars, both petrol and diesel. That is why you set the length of the float arm for the height of your vehicle's fuel tank. This will work on all factory negative earth Land Rovers that still have a factory or factory like front or rear fuel tank.

The Land Rovers that came from the factory wired for positive earth requires an NLA sender unit identified by a little tower on the top side.

When I needed a new sender unit in my SII I used a negative earth MGB gauge on an aux. panel because it is compatible with the sender units currently available and looks right (I now have a MGB tach in the hole where the fuel gauge sat). I set the float arm so the float touched the bottom of the tank when empty, used the provided clamps then soldered the two arm sections together.

You asked about the wiring. There will be an uninsulated tab that you should wire to frame ground, a wire for the gauge and a wire for a low fuel warning light. The low warning tab is easy to verify as it is an on- off switch that only conducts when the tank is nearly empty. You can ignore this tab if you don't want to add a low fuel warning light.