Are the flanges flat?
Ditto. That gasket appears to have failed due to one of the flanges being warped. The 2 options I see are, 1 - remove both the manifold and downpipe and machine both 100% flat, or 2 - buy a smog legal header (if needed where you live) with a thick flange and be done. Stock manifold is a horrible design for scavenging anyway.
