Carefully with a cutoff wheel. Usually though the weld consumes some distance so when you cut the piece off it will be smaller than what you started with. If it was overlapped I would cut to the inside of the weld.
I agree with the above. Very gently with the cutoff wheel.
When i put mine in I screwed it in place with a healthy bead of silicone. Works like a champ and when I take the trans out it is SO much easier. I just pop the cover off and I have a clear shot at the shifter bolts, rods, etc. It rules!
