Not expert, but I have used plumbers lead solder and a propane torch. If you have soldered copper plumbing pipes, it is very similar. Clean the copper/brass and use plenty of flux. I have also used a big electric soldering "wand". I have watched radiator shops do it in the old days and they had a wide propane flame they held the radiator tank over.
If yours is the early "round top" radiator, they are notorious for splitting the top seam, usually against the front support (how would I know?). I never had good luck sealing those, so would get a shop to "re-core" the whole radiator. You could also change to a later "square top" if correct doesn't matter.