Upon inspecting the battery tray I found that at some point the battery was leaking and it ate through part of that sheet metal under it. So, I'm wondering what it's called and where I might find one. Also, it looks like it's bolted on, but not sure if it's going to take some welding. Thanks in advance for any help/suggestions.
I think you are referring to the inner Fender Apron.. https://www.autokrafters.com/Ford-Torino-1970_p-9095-front-fender-apron-rh-71-73-mustang-cougar.aspx
So there is the battery tray that the battery rests on, but it bolts to the sheet metal inside the engine compartment. That's the part that's damaged.
I haven't installed one before, but I hear that some trimming and welding of the apron is required to fit.
I just installed new front aprons on my 68 Mustang -- You have to remove the fender and tire/wheel. Then drill out the spot welds around the apron-- there are a bunch ! Position the new apron and mark where the spot welds were. You'll need to sand off the protective finish where you marked and then spray with weld through primer. After everything is positioned and clamped -- you're welding the spots. Tedious but not difficult. There are videos on the web that show this work -- check them out
The tray is only part of the problem. The inner fender/apron under the tray rots out after the tray is gone. That's the part in question.