Well, now that it's all apart, there is another mounting point for the header under a cable I didn't see before.The area where the header enters the head is fairly loosey-goosey, though, so a wad of silicone in the probably won't hurt. Hope it plugs the air leak, because the carb area looks / feels tight. (Unless the flange is bunged up somehow...)
That flange would get bent on old BMW's from over-tightening. Wet / dry on a plate glass while watching TV will even it out.
Hysterical note; Indian headers are just a press fit with a tab that bolts onto the frame. You have to set your mouth just right, or it won't go in- but I never had an air leak. Weird...