How can we stop the engine from shutting off when it reaches 210 degrees.

We bought a 1968 Camaro SS at auction a year ago. When we bought the car, we learned that the previous owner had just put in a crate 350 engine. Problem: when the temp gets to 210, the engine cuts off. It starts when it cools down to 190. If we sit through 2 red lights, we are in trouble. We had the radiator, added new antifreeze. What is making the engine shut off and how can we turn this off?

We've asked mechanics here in town but all seem perplexed