Hi, my brother has a tank heater on his 684. It is plumbed with water heater hose from the plug in the water jacket on top right of engine with the heater hose thread down behind the fan to the top of the tank heater. The tank heater is mounted down between the starter and the injection pump. The bottom of the tank heater is connected to the block rain. The top of the tank heater has to be mounted below the lever of the block drain. Somewhere I have a photo of a tank heater mounted on a D-239 in a 574.