Cooling problems

Over 40ish mph is a cooling system problem, assuming adequate ignition timing.
By this speed ram air should have taken over. The shroud and fan are by now out of the picture, and the stat should be running wide open.
So you need to look to the basics;adequate rad exposure to the wind,adequate waterpump rpm, and rad efficiency.
Of course it wouldn't hurt to verify the stat is opening WIDE.
There should be,at 40 mph, a 30 to 50 degree difference in the cooling system temp between the top of the rad and the bottom. Straight water is a better coolant that any ratio of antifreeze. To run it tho, you have to protect the water-pump seal with an additive-package.
Adequate waterpump rpm starts with your engine rpm at the overheat speed. And Adequate engine timing usually includes the VA system. Most SBMopes like 44 to 54 degrees at about 2200 rpm when cruising. I have no idea about 470's but I can't imagine they would be much different.