I just went through the same thing on my 42 cab. There is actually an overlap of metal there on mine. This had allowed dirt and moisture to collect over the years and rust thru from the inside out. I used Metal-2-Metal body filler and sealed the backside afterwards with paint and such. Metal-2-Metal is waterproof and has given me good results over the last few years on the Mustangs we've restored. That is what I'd recommend, since welding would be difficult at best.
