Thanks for the pics wheelerd. I am assuming that both the heads are fed via the block and both sides are split at the water pump outlet? Being that my outlets on the waterpump and the intake mani are in use/not accessible due to the fuel line, the head outlet on the port side of the boat (right side of the motor) would be the next best as its further from the water pump then the opposite side? Thinking about it, the water in the motor is regulated by the thermostat so I would have thought most of the outlets will be fairly similar in temp anyway?
I havn't got round to building the heater box yet, but I will post pics when I have made it. Still trying to work out the cheapest way to blow air through it. Was thinking of looking into a few high flow computer fans or something similar. Need to do a bit of testing for it.