You could get a set of ls6 heads. They are the same as the 317 heads your have but they come with a smaller cc chamber. It is 64cc compared to the 71cc of the 317 heads you have. The casting number for ls6 heads is 243 or 799. You wouldn't have to mill as much and still have some room to mill more if the heads become warped. They also have the same size valves as the 317 heads.