I hate that kind of header flange. Never had much luck with them, even using the aluminum or copper gaskets, they ALWAYS start leaking. I'd be tempted to cut both flanges off and use a band clamp instead:
http://www.summitracing.com/parts/sum-693250-1/overview/Ball flanges also work really well. If the band clamp doesn't work, go to an exhaust shop and have them put a ball flange on it.
