Headers can get pretty darned hot, like 400-450* near the port. If the engine is running lean or the timing is off, they can get even hotter than that. Check your tune.

If your overflow tank is overflowing after shut down, that is sometimes a symptoms of a cracked head or blown head gasket. Go down to your local Autozone. Borrow their coolant system test kit. Follow the directions, and in about an hour or so, you'll have your answer.