high speed overheating

Have you always had the problem?

If the problem is new, i don't think carb tuning would be the problem. As mentioned a lean engine will run hot.

Radiator could be clogged...thermostat could be stuck...or a collapsed radiator hose (like mentioned above).

The other thing I would look at is the water pump.

I don't think "hard block" is a good idea on a street car...you reduce the amount of coolant in the cooling system and old cars aren't known for their wonderful cooling abilities to begin with.

Paul