Battery Box Issue

[37Flat]

I just started stripping stuff off the firewall when I hit a snag. Apparently the battery box is not only held in by rivets but 2 welds. In the picture you can see them where the paints knocked off to the right and left of the center hole. I searched the site and found others with the same problem, but no solution on how to break these welds without damaging the fire wall. As the welds are internal (how'd Ford do that?), all I could figure was to drill them.

Does anyone have a tried and true way of getting the box off?