I was hoping that some of the Gurus would throw in two cents. Since I'm about to go through this.
I had also thought of riviting or screwing it into place short term since It looks likes its going to be quite difficult to weld with the engine in. I'm planning to pull the engine next winter and paint the engine compartment, fix the engine shelf (cut for A/C) and replace the tin and wiring.
My plan was to paint the battery tray out of the car, and remove the paint from the welding surfaces, replacing it with weldable primer, and temporarily rivit it in.
I'm in no way suggesting that my plan is the right way, only way, preffered way, in fact, I don't even know if it will work at all...

But that is my plan.
FWIW