To match port on the exhaust side, use an exhaust manifold gasket as a template. Once done on the head, you can check the exhaust manifold and make sure it's going to match the head. It's OK for flow if the port running is smaller in diameter than the exhaust manifold but not vice versa. You can grind the flat spot on the floor of the exhaust port at the outer edge but take care as you move in or you will end up in the water jacket. This is one aspect of porting most DIY people can do.