I think what you have found is the reason tube headers are notorious for not sealing. I tried to straighten the headers I have on my 292 now by grinding down that area on the flange. That area is the weld and if you grind too far you have little or nothing holding the tube on depending on penetration. I have another set of headers to replace the ones I ground. 8-[ I'm thinking of doing what you have already thought of. I'm going to try and MIG some more material around the existing weld and then flatten with a belt sander. If someone has a better idea I'd sure like to hear it.
Larry
