first problem would be......how are you bolting this flange to the head in the first place?you'd have to first cut this "header/cat" off,grind/resurface the area to make it PERFECTLY smooth, make a gasket, make a flange and then somehow connect it to the head.welding is probably out of the question since its cast, and that wouldnt be very reliable in most cases, especially with the location of this and the extrememly high temperatures it will see. and in order to actually bolt it on like usual, you'd have to drill into the head, tap the holes, place studs, and then bolt on like normal. so this is going to be harder than you see it as being
btw, its not bong........its bung