I am not totally sure but I think the tubes were brazed up as a complete entity and the ally was cast over it making it hard to restore.
If I were in your shoes I think I would cut off the legs and machine back into the ally and let in new stainless tubes glued in as you suggest.
Bear in mind that when running the tubes get very cold and when parked up they get very hot when the heat transfers back from the motor so be well advised as to the glue you use. The aluminium expands and contracts so much more than the ally.
Another option would to be to leave a stub and glue the new tube over the top. May possibly be detrimental to the breathing but, to be honest, its not the best designed inlet tract I have seen. Make sure you have the bends right to reduce the stress on the new join as much as possible.
Another option would be to make new tubes from ally and TIG them on.
Hope that helps