Given I'm not a gasket expert but having had poor experiences with aftermarket manifold gaskets your solution seems extreme especially since this problem is not prevalent. True, the oe gasket is not cut friendly so you can't rush it. The safest method would be to sandwich the gasket tightly as close to the cut line as possible. Expose only the material that is to be removed at each runner and then grind out with 1/2" bit or stone(s). Otherwise a copper gasket would be my next choice. They seal well and can be more easily opened up. I think Joe@ZSpeed sells them.