If the mount was broken for a long time it stands to reason the the hard movement of the tranny and then sudden stop at the end of the mount travel could have broken the case. Aluminum fails from fatigue especially cast aluminum. TIG welding aluminum can be pretty strong - from my experience, but I would make sure you found someone that had experience with cast aluminum.
Definitely take the tranny out and appart before you weld it. Aluminum must be thoughly cleaned and prepped before welding. The outside surface of the aluminum oxidises (probably spelled wrong) very quickly and then makes it hard to weld. It should be ground or sanded right before it is welded.
Good Luck
