i would just open it up width wise on your pic. leave the same amount of space on the sides as is on the top. don't know how much your going to gain from this other than a little bit less back pressure. gastket matching a set of cylinder heads will give you another 5-7 horses if that on the intake ports. on the exhaust, i doubt it will do anything. the main reason for matching is to be more efficient. i've been porting cylinder heads for a few years now with a guy who has been for 25 years. we do mostly circle track racing applications but what ever you want done we can do. from diesel tractor heads, small block, big block whatever. cast iron or aluminum makes no difference. anyways back to the port matching, i have flow tested heads and stuck my finger in the port partly just to see what happens and it just disrupts the flow about 2-5 cfm.
hope this helps
nick