There are two plugs which need to be replaced with pipe fittings onto which half inch (internal diameter) heater hose will fit. The one where the hot water comes out of the motor is between the two rear head bolts (of nine) on the left side of the rocker cover. The other where water returns to the engine is in the top of the housing that is bolted to the right side of the thermostat housing with the bypass hose from the water pump connected to the bottom. Worth noting that the plug where water temp sensor is fitted is on right side of front of head under thermostat housing.
The threads on these are three eighths British Standard Pipe ie common water pipe thread in many countries including Australia. Note the size description is the approximate bore of the pipe, not the larger diameter with threads of which size is critical. Nineteen threads per inch. (American NPT threads around similar size have 18 TPI)
The setup for the Land Rover optional heater on Series 2's is with an on off tap on the head with the control knob on top and the outlet at a right angle which reduces the length and amount of bending necessary in the heater hose. Worth checking that any heater tap allows full flow of water. I discarded one of the type that cable control can be connected to that was installed between hoses on my S2 because when full open it was obviously a significant flow restrictor.
Worth noting that on the 300Tdi motors in Disco 1's and Defenders, with a somewhat similar setup with water going to the heater from the rear of the cylinder head and returning to the front of the motor, there is no heater tap. Seems the heater circuit is regarded as an important aspect of the cooling system to remove heat from the rear of the motor. Note that some cars have fancy heater taps that allow water flow to continue but bypass the heater element when heat in the cab is not wanted. I installed a bypass tap on my Series 2 that can be turned on in summer while the heater tap is off for the same result..
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