Just because I think it might leak. You have a good sized engine (to go with your boat size I think) and I doubt there isn't enough power in the coolant pump to purge the heater lines of air. I'd run the engine up to temp. first(with mine I'd leave the radiator cap off the manifold), and just see if you have good flow through, top off, and you could be fine. One of my lines goes below the engine to the water heater, no problems with air.

If you end up with an air block, then you might have to figure out how to vent it somewhere. I'll be interested in hearing how well the radiator works. I should do that, "free" heat in Maine.