You don't have to modify anything to use donut gaskets, if that's what you like. The rear header has a slight taper at the inside of the flat mounting area. That taper will accept a doughnut gasket. The front header has a flat surface, but there is a doughnut gasket that can be used on that area. It is a half doughnut-half flat made out of the doughnut material. It has a metal clip to hold it in the header, centered up nicely for installation. I have both on my truck and once you have used these kind of doughnuts, you'll never want to use anything else. The better muffler shops have them in stock.
