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On the flywheel end of this Cylinder head on the picture you will see the word CRAB written. Right behind the letter B of Crab is a steel plate with 2- 3/8 bolts holding it on with a gasket underneath to the back of the Cylinder Head . I have seen some of these plates with threads tapped into then for either a 3/8 or 1/2 inch 90 degree elbow threaded into it.
On the front end of the engine you will see one pipe plug visible on the top of the waterpump housing. You could possibly use this plug also or the one that is visible on the side of the Thermostat Housing. Heating these pipe plugs with a torch in the dead center and then quickly tap in socket and plug will break loose easily. I seem to recall that this is how I seen these units plumbed for a heater in years past.
Hope it helps .
Simon C
Simon C