I remember hanging around a radiator repair shop when I was 12 & he was always using a roll of solder and an oxy/acet torch. Not sure what % but I'm sure it was not silver solder & I'd think SS would not be strong enough for that but I am WAG. Last year I did have a bung installed in a new gas tank & everyone said to weld it that solder would not be strong enough but the shop insisted otherwise so I went with their thoughts on that & it was rock solid/no leaks. I would say either 60/40 or 70/30 (not sure which) acid core flux solder Oh & now I remember as I type this that he was always brushing on something with a mini brush with a chrome handle what I now assume was flux to clean the joints. He'd heat the joint then brush the stuff on then blow it off with shop air & use a wire brush before or after & this'd expose the crack then he'd solder it with the roll of solder in hand & torch then dunk it in the water tank & pressure check it for bubbles