Exhaust manifold leaks

You don't really need the heat riser in Vista, CA, so simplest would be to plug the holes with a screw, as suggested. If you don't have a tap, or can't swing it with the manifold on the car, maybe a thread cutting screw would work. It doesn't have to be strong. More important is that the cross-over passage is not plugged with carbon, as it was on my 273.