I use SS wire to help keep the rust from growing so fast on the bottom side of the welded area. plus its a stronger metal so when I grind off the weld it more likely it wont crack or when the car is done the twist in the body from a well built motor wont crack the weld, and cause the filler to pop or crack.
I'm going to use frame connectors and soled body mounts to help on this.
the car is going to be black, so I got to make the roof as straight as I can.