If I remember correctly, doesn't the Perkins have 2 square drive threaded plugs at the top front of the engine. Delivery to the heater comes out of the cylinder head, and the return goes in just below the thermostat housing. Haven't worked on 1 in a while, so this ain't gospel. Westerbeke have a nice neat double-T fitting, with a built in diverter reed that may help you.
I have a different engine, but I came off the thermostat and the engine water pump. If you come off the heat exchanger I would think you would have to hook in before the heat exchanger as it will be cooler after it leaves the exchanger and may not heat as hot, but it may not matter. Just a thought.
That derelict boat was another dream for somebody else, don't let it be your nightmare and a waste of your life.
Yeah! it is as I remembered it to be. My comment about the cylinder head feeding the heater was because that would be where the hottest liquid exiting the engine to the heater element would originate. The T in the photo has his temp. sender unit at this location. No real thermo-syphon effect, so yeah, either hose can be in or out.