A friend's '74 Plus 8 was having overheating problems and diagnosis showed a number of issues contributed to the problem: Front fan incorrectly fitted, recovery system not used correctly (air in system), leaking radiator plugs and insufficient coolant flow through the triple pass (racing) radiator.
Triple pass radiators need a greater flow than standard radiators. With the thermostat removed the car ran at correct temperatures but with the thermostat fitted, it ran hot under load (high speeds/steep inclines). It is important to keep the thermostat in a road car so I started looking for solutions and found that an Australian company, Tridon, now offer high flow versions of their thermostats providing 30% greater flow of coolant. In fact these are now the recommended replacement for the early '70s Range Rovers (Part # Tridon TT 2000-180). This photo shows the obvious difference between high flow (left) and standard (right).
I found fitting the high flow thermostat along with rectifying all the other issues fixed the cooling problem and the car is now running at correct temperatures.