I would not try to tig weld over a brazed joint if I were you. First of all there is no need to do so, and second, you will most likely mess up the tig weld because of brazing contamination trying to get into it and you may also mess up the brazed joint as well. If you want to mill off the short rib section over the monoblock and overlay the rear of the new rib over it, that is O.K., as long as you use heavy duty solder such as Brownells Hi Force 44 to stick it down. Really there is never a good reason to do any welding on the monoblock. Those halves that you spoke of are brazed together in an oven and are as strong as most welded joints because of the amount of surface area involved. Bob