Definitely check the gasket, but also check the flanges. I had a set of bbk shorties that the flanges were tweaked on (one was slightly bent up and another slightly down), it was a mfr defect and Summit replaced them. I checked them by placing the flanges flat against the wall and noticed 2 of the flanges did not sit flush with the wall and the other 2 did. It's not the most scientific method but it was effective for me. The Macs are notorious for warping. Check the seal between the flange and the header tube too, I had a set of longtubes where the flange and tube weren't welded properly (there was a gap), another mfr defect and Summit replaced those too.
Another thing you can try is to double up the gaskets.
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