The following is from the Ed Parks website.
BATTERY TRAY REPLACEMENT
Q: I was wondering what is the best way to replace a rusted out battery tray?
A: The battery tray is welded to the car in 3 locations. This area and the part that makes a 90% turn seldom has any damage. If you trim the old tray, leaving about 1" of good metal where it is welded, and then trim off the new tray to overlap, you can use sheet metal screws to fasten them together. Works perfectly. If you don't do it this way, you will need to remove the seats and rear carpet panel and insulation from inside the car, because you will start a fire when welding
to the bulkhead. Hope this helps. BTW, we have done it both ways, and NO more welding, NO way.