whats the best thermostat for /6

Thermostats do not make your engine cooler if you have an overheating problem it usually not the thermostat.

Thermostats keep your engine from running too cold, not too hot.

You probably need to flush your cooling system. I would also make sure that your pump is moving water like it should.

Also:
Check your plugs to make sure your fuel/air mixture is correct.
Check your radiator hoses aren't collapsing at high speeds.