first thing I would do if it hasn't been done(allways start with the obvious) check or replace the resevoir cap(radiator cap), if it doesn't have enough spring pressure it will allow excess coolant out of the engine into the over flow, casuing it to overheat. As I'm writing this i think of something else to check, Do a compresion test!!!! The 470's have unsupported cylinders where they seal on the head, they can and will leak cylinder pressure into the cooling system, usually accompaning low performance, skipping and coolant mixing with the oil( but not allways). One other thing that pops into my head is the exhaust riser, if it is clogged, it will not let enough raw water out of the system for cool water to replace it, especially when running higher speeds, check your exhaust bellows for signs of running hot. Pull the riser and flip it over, inspect the water outlet holes, hook a garden hose to the riser and see how well the water flows out of it More ideas keep coming up, did you check the water pick ups? remove the pickups and make sure there arn't any leftover water pump impeller pieces in there. I ran into a 3.0l that was doing the same thing last summer, tried everything, finally pulled the water pipe and hose out of the transom plate, it had a piece of an impeller lodged in it, we had back flushed the line several times, but untill we ran a snake thru it, it never budged. There are a number of things to check, these were the first that came to me that I've seen in the past causing overheating. Good luck
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