Remove all water from the block

I would aim for 70% glycol for lower range before it freezes. He won't be over-heating in Vermont anyway. Don't worry so much about heat xfer capability of the fluid. There are many factors involved. The main "choke point" in heat transfer is the radiator to air interface, so if the coolant flow keeps up with that, it isn't the limiting factor. You could put the biggest water pump known to man and you still wouldn't get past the radiator bottle-neck in rejecting heat. Hope that is clear.