My '46 1/2 ton had a few pinholes in the same area.. back of the cab in the "beltline" area.
I saw an old school dude on American Hotrod do some body work with lead.. and I just happened to have some old lead battery terminals laying around.. so after sanding the area down to bare metal, I applied a lead patch, sanded smooth, and painted over it.
Lead ain't gunna rust or bubble.
I suspect this is simular to the solder method mentioned earlier in this thread.
