Overheating headache

Forget the headers being a source of the problem. You are already on the wrong track. They are not the issue. When you accept that fact you can then get on the right track to diagnosing things by focusing on cooling components. The coolant is possibly not circulating properly. That’s where I would start. Thermostat, flow blockage. Start with the basics as laid out in a factory service manual. You got one of those??