I believe that the W123, W124, and W126 (probably all MBs) have a pump in the lower right front which pumps the coolant through the motor into the heater core while the motor is off. So if you're tapped into that same side of the thermostat then you'd be able to give your WVO tank a little more heat from this.
What sort of suggestions are you angling for? Your model 1 appears right on to me. You'll just have a hose whatever size OD your fittings it Ting into the current 7mm feed fuel hose for your WVO's return. The diesel return is automatically correct because of your setup.