Big Block high speed over heat

I have a 1968 Camaro with a 1969 396 hsp big block. I have a 180 degree thermostate in it that runs at 193 degrees. When I hit the hgihway 55 mph or higher the temperature rises above 200 degrees. I have had the radaitor and cap checked at Ann Arbor Radaitor, no problem. Replaced water pump with high volume pump. Heads have been redone with hard seats to burn the unleaded gas. New Elderbrook carb and aluminum intake. Still the same results.

Any ideas.

Thanks