I have had no luck with the copper gaskets at all. They are way to hard. I find that the flanges are made very poorly on the headers. Run a file accross them to flatten them if needed. I like to cut the flange so each tube is independant from each other and use Felpro gaskets.

 

file the flanges as evenly as possible, apply hi-temp rtv, & carefully install. i've done it that way for years, on the street, no leaks.