280, skip the metal melting repairs on radiators. They're seldom truly effective. I've seen many attempts, and most are failures.
You can try stitching, but that's a nighmare. It involves drilling the crack and threading in plugs.
The best way to fix them?
High strength JB Weld epoxy. It will work up to 600 deg. F and take a LOT of pressure. Just fill the cracks according to the directions, leave cure, and then pressure test.
Oh, and you don't have to buy new radiators.
Hit an architectural salvage yard, or even a scrap yard, and you can often get hot water radiators for $15 a section, or less.
Just make sure that you get hot water radiators, NOT steam radiators. Yes, there is a difference. A hot water radiator will work on a steam system, but not the other way around.
Also, if you decide to go separating sections, you may run into problems with leakage. There were a lot of radiator manufacturers around over the years, and many used very different methods of sealing the sections, from crush nipples to section washers to sealant compounds.
A high temperature silicon sealant can be your friend in ensuring leak free joins.