Maybe someone else would comment on this, but if you are to follow Brad's suggestion and weld on the inside, will there be any benefit to a large alloy plug in the tube and maybe a (split) ring on the outside under the flange, both to act as heat soaks?
I hope I am not trying to teach you to suck eggs, but think about if it is necessary to clock every flange you weld on relative to any others either on the same part or on a part to which it joins. You might need to keep very good track both of which part joins which other part and the clock position at which the two parts touch.