This is a new one, is pipe porting something a shop or builder is pushing on you? Hold your gasket against the port make sure that the opening in the gasket is not overhanging the exhaust port, now hold the gasket against the flange on the pipe, use a magic marker and mark the points that that are smaller than the opening. Use a dremel or small file and taper the opening to be larger than the gasket opening. Check your rules